Now, I'll give you a quick look at the resolution of the 50D. What I've done here is photograph my world class test setup using the same lens on three different camera bodies. The comparisons are the EOS 5D, EOS 1DsMK II and the EOS 50D.
The lens is a Canon EF 70-200 L IS (very sharp lens) with the IS turned off. I'm using autofocus on each camera, shooting at f8 with exactly the same manual exposure on each body. .6 sec exposure time, ISO-200 and the zoom set to give the same field of view on all cameras (as close as I can get it.) The only adjustments made were to equalize the white balance (using the white card in the center of the frame), exposure highlights and shadows set to no clipping in Lightroom. There is no sharpening or any other adjustment to any image other than the conversion to jpg for web viewing. The conversion was done using Adobe Lightroom 2.1.
Here is the master image. I'm cropping on the Quick Reference card in the lower center of the frame to compare the resolution of each camera.
Master Scene (EOS 50D version of the shot)
EOS 5D
EOS 1DsMK II
EOS 50D
The crops are different sizes due to being cropped to 100%, the main image files are different resolutions. The 5D being a 12 mpix camera will have the smallest crop, the 1DsMKII will be the largest and the 50D will fall somewhere in between.
If you look closely at the large dial on the quick reference card, this area is where the absolute resolution of each camera can be observed. I set this up to the 1DsMKII (at 16.7 mpix) will give me a good solid view of the detail in this area. What is happening is the MKII is doing great, lots of detail in the rim around the outside of the dial, and other small text printed on the card. Next, comparing the 5D and 50D, you'll see that both are very close in absolute resolution. The 50D is pulling just a tad more detail though, enough to see the open space in the circles and the finer text is slightly more readable.
Another note. You'll hear tell of pixel level sharpness on the internet forums. Don't get too worked up over this term, because what it means is how much contrast the image has at the pixel-pixel level and is a function of how much high frequency and low frequency image information is contained in each pixel. In the end, shooting with a raw file, none of this makes any real difference, mainly because you have control of the contrast, and detail in post-processing. You can adjust the sharpness of these images to your taste. No matter how much you adjust any image, you can't extract more detail than was recorded and based on what I'm seeing, the 50D is recording more image detail than the 5D by a noticeable amount. In the real world, you won't see this difference on any print up to 16x20 inches, with the exception of the MkII, as the detail is definitely more defined and will show up at that print size and possibly even smaller. Out of the camera, raw files from the 50D will look a little softer than their full frame counterparts. The detail is there, post processing will equalize anything. In fact, I'm finding the 50D files have a great deal of latitude in post processing. Just don't expect to be using the same adjustment settings as you will on other cameras. It's a different beast and will require a different sharpening setting.
The bottom line is this. Canon has created a APS-C crop sensor that can deliver more detail than their 12 Mpix full frame sensor, with noise levels that are within 1/3 of a stop at ISO 1600 and above. When you think about it, it is amazing how far things have come in just 3 years. I'm very interested in seeing what more can actually be done with the APS-C sensor. They may have come very close to if not reached the limit of the technology here. I don't know that anything short of better lenses is going to pull more detail out of these sensors in the future.