Well, at this point I'm fairly heavily invested in the Pentax system so I'm extremely unlikely to switch. But, I do keep tabs on the others so I like to think that I'm at least
sort of well-informed.
The Pentax system is, surprise surprise, not perfect. But none of them are, that's why there are still five companies moving enough DSLRs to keep them doing well. (I'm ignoring Sigma and Panasonic and Fuji here.)
What I can do is tell you why, if I were starting over today, knowing what I know now, that I'd have to go with a Pentax because I couldn't comfortably buy into any other system. If you are easily offended, stop right here - these are my own opinions mixed in with a few facts.
This is also mostly looking from my own perspective, where I'm willing to drop maybe around a grand for a body and consider $750 to be the most I'd ever (?) be willing to drop on a lens, and that's still a ton more than I like to spend.
I wouldn't buy a Canon because I don't like the feel nor build quality of their entry-level models and the mid-grade (40D/50D) ones lack proper weathersealing and still don't quite feel right. (And sorry, I'm not willing to spend ~$3k+ on a camera body, folks. This is just a hobby and I'm not rich.) In terms of lenses, lots of stuff is available but I feel that Canon artificially keeps quality levels low on their cheaper lenses in order to sway you into buying their big-dollar "L" lenses. IS generally is only available on high-end, expensive lenses, and off the top of my head, I don't think they have a single prime with IS. Canons also seem to have a more "plastic" look than some other DSLRs, which some love and some don't. The all-electronic nature means concerns about "chipping" old lenses that don't exist in other systems. In all honesty, I also have an intangible distaste for Canon as a company, partially stemming from my habit of rooting for the underdog - this is purely personal, but at this point, I would be unlikely to buy anything with the Canon name on it. On the good side, their AF is quite good, their high-ISO noise levels are generally good (though not class-leading any more and never were
much better than the rest), and you can mount mount most any old lens on there with an adapter - but you have to make sure to get one with focus confirmation and stop-down metering.
I generally like Nikon but wouldn't feel comfortable buying one. The entry-level ones can't AF old lenses and can't use really old lenses hardly at all. The D80 was getting a little old but the D90 is pretty impressive on paper (forgetting the somewhat silly "video" thing.) The D300 is great but overly large and heavy and quite expensive. The D700 is even better but that's a
lot of money (even if it's a bargain compared to the D3) and I'm not convinced than FF is worth the extra cost, size, and weight. The AF system on the D90 is a bit stripped-down so I wonder how it will perform next to the competition. The D90 also lacks weathersealing. But lenses would be a big problem. Nikon has some pretty amazing new zooms by all accounts, but they've almost abandoned primes (although IIRC they do have
one prime with IS, which is more than Canon), and many of their lenses are pretty old designs. In fact, outside of a few highly-regarded zooms, there's not that much that interests me in their lens line-up. Furthermore, they have the worst adaptability of any of the big DSLRs; forget trying to adapt older lenses.
Olympus makes some very interesting DSLRs but their strength is also their weakness - the 4/3rds sensor is right off the bat the wrong aspect ratio, but also will forever have higher noise levels than the competition. You'll probably never want to go beyond ISO 1600 and even then you might be pushing it. The E3 does have weathersealing but is a little more expensive. Their AF is reportedly very good. Lenses are a very big concern - there are fewer than for any other system (and less third-party support), and while the quality level is apparently quite good across the board (with several
great lenses), the costs are rather startling. You're looking at a ton of money to get a decent lens collection going. On the positive side, they also have wonderful lens adaptability, like Canon, but again, you have to watch for adapters with focus confirmation and stop-down metering. I'm a little fuzzy on IS with older lenses though, I think a recently firmware update changed the settings, but I think that originally, it couldn't do IS with older manual lenses. Still, in-body IS has to be listed as a positive of the system.
Sony doesn't interest me because they just don't seem to be taking advanced amateur photographers seriously. When the A700 came out, you couldn't turn off high-ISO noise reduction, and the photos suffered greatly for it. With the A350 and A900, they've proved that they're willing to sacrifice image quality for megapixel ratings. The A350 is basically just an A300 with more megapixels and lower image quality. How many DSLRs actually list the megapixel count on the side like you'd see on a $150 PnS? The A900 has 24mp and much worse high-ISO performance than the D700/D3, and almost certainly worse than the 21mp 5Dm2. (The 5Dm2 goes to 25,600 ISO and the A900 stops at 6,400, to give an idea.) But lenses are another big concern. There seems to be a mix of old, rebadged Minolta lenses with varying quality and some new,
very expensive lenses. If you're not rich (or making a living at it), you'd probably better stick with third-party lenses. Lens adaptability is also pretty poor, you're unlikely to mount anything but current Minolta/Sony lenses on it. Another problem I have with the A300/A350 is the emphasis on acting like PnSs, encouraging you to compose on the LCD instead of the small and dim viewfinder, etc. On the positive side, their AF on the newest models is supposed to be very, very good, and they have pretty good in-body IS.
Pentax has its negatives, too - the AF is improving every model but unlikely to be at the level of the top models out there today. (One rumor is that the upcoming K2000 is the fastest yet, using the same setup as the K20D but with software tweaks, so hopefully those'll appear in a K20D firmware update.) Don't get me wrong, the K20D is very fast and works well in low light, but probably not
quite as fast as some others. There aren't as many lenses and third-party support as C/N. There's still question about if Pentax will release a FF camera (the current thinking is that it's still being decided, but their true medium-format DSLR is basically ready to go and they just need to decide if it's financially feasible) but that doesn't concern me that much, I think the appeal of FF for most people is more emotional than rational (not that there aren't advantages, but there are trade-offs, too.) It's harder to find Pentax cameras and equipment in local stores than any of the other lines. If you want high continuous shooting rates, look elsewhere - 3 fps is it for any model unless you count the video-like 21fps of the K20D (no focus/exposure adjustment during this rate, and smaller photo size.)
On the positive side, all their models are weathersealed now (which I've found to be tremendously useful over the time of owning the K20D and a weathersealed lens), you get a
lot of camera for your money, the build quality and feel is generally the best in the price range, they generally give the photographer good tools to work with (they're always had spot metering and DoF preview in all entry-level DSLRs unlike C/N, they all had top LCDs until the new small K2000, and their kit lens is in a whole different league than C/N/S's comparable lens.) The lens selection is pretty solid at this point, and Pentax has been churning out new lenses fairly regularly. There also are few clunkers in the line-up, and many lenses that are really great. They also continue producing new prime lenses like no one else. They generally have nice, bright viewfinders and good in-body IS. The lens compatability isn't as wide as Canon or Olympus but the M42 lens support is the best, and you can find an astounding variety of great old M42 lenses out there, and all automatically get focus confirmation, metering (you can just shoot in aperture priority and it'll adjust, or shoot full manual if you want), and IS.
The K20D with a 16-50mm and 50-135mm is a great setup IMHO. The 50-135mm is really a nice range, basically the same as 70-200mm on a FF or film camera, but it's about half the weight (or less, compared to the tanklike Canon) and shorter, so it's very easy to handhold. Both lenses are weathersealed so you can shoot in driving rain with no concerns. (I went down Splash Mt with mine and kept it out and shooting all the way down the big drop and through the splash, no issues except water splashing hard enough to get past the long lens hood and onto the front element! No big deal...) 135mm is also, IMHO, long enough for most telephoto shots, especially at Disney. And I think that 50mm as the starting focal length is
much more useful than 70mm.
However, switching systems can be a bit of a pain, especially if you have a pretty modern DSLR as you do. Have you considered just going third-party? Tokina does sell a variant of the Pentax 16-50mm and 50-135mm (with a different barrel, no weathersealing, and inferior lens coatings) but I don't think they're available for the Sony mount. Tamron and Sigma both have good fast wide primes, and Sigma makes a 50-150mm F2.8 however again, I'm not sure if it's available in a Sony mount.
And it's wonderful, although heavy so it's only particular places it goes. I looked at a CZ 24-70 at a photo shop and tried it on an A700. All I could say was WOW! I had hoped to also look at the 70-300 but haven't run across it anywhere.
Apparently, they put an "unobtainium" lens element in there. 
Regardless, the 12-24mm seems to get better reviews (whether Pentax or Tokina) than the 10-20mm last time I checked (admittedly, I don't pay too much attention to non-fisheye ultrawides.) Actually, some comparisons have shown the Pentax 12-24mm a little better in overall sharpness than the Tokina although they theoretically should be about the same.
but I do agree with his points on the Pentax camera's strengths (weather sealing, AS in the body, etc.). However, after owning my K200D since May, I have found some shortcomings. The biggest is the K200D does not like to autofocus in very low light. It tends to hunt a great deal and sometimes just flat out refuses to AF. This is not a deal breaker for me as the MF capabilty is easy to use. Also the FPS is not the fastest. 99% of the time this is not critical, but it would be nice for some of the sports stuff I like to do.
