I have a question for those of you who have the digital expertise: to what degree is the limiting factor the lens's resolving power/sharpness, and to what degree is it the sensor? To ask it another way, if one is using a "consumer" (i.e., not $1500 pro glass) lens, would the sensor resolution (6MP vs 10MP) be all that significant?
You'll get quite a lot of different answers to this question because it's really not black and white. I would certainly think that the new 12mp sensors for point & shoots are a total waste, but it's a bit trickier with DSLR.
The first problem is that some consumer lenses are very good. The classic example is the standard 50mm prime. It's extremely simple to build a really good one. In fact, it gets harder the further from 50mm you go and the more zoom range you add in. A cheap 50mm prime is likely to outperform a really expensive wide ranging pro zoom at 50mm in every way.
The next problem is that even cheap zooms can often shoot OK at some ranges and apertures. If you take a typical zoom in the middle of it's range and shoot it stopped down a few stops (say f11), it will usually shoot quite well. I would be that if you compared a Canon 70-200 f2.8 lens (a really nice pro lens) shooting at f2.8 at 100mm with a Canon 70-300 f4-5.6 (a decent consumer lens) shooting at f11 at 100mm, they would probably perform similarly.
Finally, one of the biggest problems in getting the most from a camera setup isn't just the lens but the photographer. Lot's of technique errors often detract from getting the best sharpness. A lot of people use the reciprical rule for picking a shutter speed, but that's intended to get you to good enough, not critical sharpness. A lot of people focus on the wrong part of the subject or use focus lock on a moving subject. A lot of people shoot tripod shots but still push the shutter button manually or don't use mirror lockup.
Another problem with extremely dense sensors is diffraction. The smaller the pixels on the sensor, the sooner diffraction starts to become a problem. This is one reason why when comparing two sensors with the same resolution, the larger one can sometimes take better pictures. The effect of diffraction gets worse the smaller the aperture. It starts to become an issue somewhere around f11 on a 1.6x crop sensor and gets progressively worse as you stop down. This is a problem that cannot be fixed with better glass or better sensors, only wider apertures or larger sensors.
Finally, if everything else is equal, you will get more noise with a higher density sensor. If the don't change anything about the sensor design or image processing and simply pack more pixels on the same size sensor, you'll get more noise. Of course, everything else is rarely equal, so as they have increased the density of pixels on sensors they have also done a good job of decreasing noise as well.
That's probably a longer and more boring answer than you want. My simple answer is that after about 6mp, the law of diminishing returns sets in and improvements in megapixels start to matter a whole lot less. That's partly because of the limitations above. The other reason I stress that is that most people mentally compare pixel numbers linearly when they should compare them exponentially. When most people hear that the new Rebel XTi is 10 megapixels and the original Rebel was only 6 megapixels, they have this gut notion that they'll have almost twice the horizontal and vertical resolution. In fact, the pictures go from being 3072 pixels wide to 3888 pixels wide or an increase of just over 25%.
Excluding the old 3 megapixel Canon D30, I don't think that megapixels should be that big a factor in selecting a DSLR. For my shooting style, I'd put having more megapixels behind noise levels, autofocus speed, autofocus accuracy, autofocus coverage, ease of use, durability, buffer size, write speed, metering accuracy, or spot metering ability.
One other thing to consider is that if you camera is capable of shooting at a very high resolution and you want to shoot RAW, you can't throttle back. If you want to use smaller file sizes to fit more pictures on a card, shoot faster, or use less disk space, you're out of luck. Your only options are to either shoot full resolution or switch to JPG and giving up all the benefits of RAW. That's why a 1Ds is great for landscape shooters who take a lot of time getting each shot just right and a 1D is better for photojournalists.
OK, now that's way too much info.
