Want to hear something good about Olympus? They're not Canon!
Kidding, kidding.................
What about the Pentax line? They offer in-body IS and
IMHO have a more attractive offering than Sony or Olympus in terms of price, lenses available (and price of them), and features. Your mileage may vary, of course.
The things that make the Olympus line unique are...
1) The "standard" 4/3rd sensor. This is, as the name implies, a 4:3 ratio like a point-n-shoot camera, not a 3:2 ratio like a 35mm camera or any non-4/3rds DSLR (ie, everyone except Olympus and I think Panasonic.)
2) The 4/3rds sensor is, by design, a bit smaller than the APS-sized sensor in most other DSLRs. This allows the camera and the lenses to be smaller (and gives you a 2x crop factor instead of the 1.5x-1.6x on most other DSLRs) but it also means more noise at higher ISOs. The E510 specifically also seems to have some strange noise settings, I've seen in a few reviews that you need to fiddle with the settings to get the best results in terms of noise.
3) Some have Live View available, which lets you use the large rear LCD to frame your photos, like a point-n-shoot, but with limitations on focusing, etc. It also leads to a smaller, dimmer viewfinder. I've gotten the impression that most people will end up sticking with the viewfinder but obviously that's a personal choice.
The good is that they supposedly have one of the best kit lenses, comparable to the Pentax - ie, metal mount, "true" focus ring (not rotating the front of the lens), and a non-rotating front element, which makes it... ummm, I have to phrase this just right...

it makes it
easier to use a circular polarizer. Good optics, too.
And, it's certainly capable of excellent results, like any DSLR.
I'm not sure what the lens adapter situation is, but in terms of 4/3rds lenses, you're fairly limited because the mount is so new, whereas other DSLRs will generally give you a pretty wide selection of legacy used lenses to choose from.
I'd also be concerned that the E510 is so new, that you're likely to pay premium prices for the next couple months. Beach has it for $800 - that's more than you can get a K10D for (after rebate). That's a lot of money for the class that the E510 is positioned at - I bet it'll have some decent drops in price soon. The older Olympuses are selling for no more than $500.