The first shot is from the 50D and is cropped at about 75% (down rezzed in CS3 to the same pixel size as a 1D3 image) so it should show more detail and less noise then the second shot, a 100% crop from the 1D3. Both of these were shot with the 400 f5.6L at ISO 400 with no exposure compensation. I converted the RAW files with Canon's DPP with no sharpening or noise reduction so you're seeing the RAW data.
Look at the noise!
I noticed the default noise reduction in DPP is way up there for the 50D and almost at zero for the 1D3. When noise reduction is applied to the 50D files the noise disappears but at the expense of sharpness and the files lose any of that "POP" I'm used to seeing with the 400 f5.6L or 70-200 f4 L IS. I'm not a big high ISO shooter but ISO400 c'mon Canon!
As it stands right now I could crop or uprez my 1DmkIII files and they'll still look better the the 50D files unless they are shot perfectly. I went through that in the past "exposing to the right" with the 1D and 20D but it just wasn't needed with the 5D and 1D3. I love the exposure latitude of the 1D3. I'm not real excited about files I've got to PP the hell out of when I don't get the exposure perfect!
Walt: The 18-200 isn't all that great IQ wise on the 50D but it might be OK on a 40D. It's got a decent build, great IS and fast AF. Good do it all lens.
Bakes: Of course the high ISO quality from a full frame sensor is going to be much better. I did notice though that people are claiming the D700 is still not quite as good as the old 5D but that's probably just splitting hairs.
Basically AFAIC Canon is now doing what Nikon has always done (up until the D300, D3 and D700) which is smear the image with too much in cam noise reduction. The photosites on the 50D are down around point and shoot size and it appears all the trick processing Canon built into the digic 4 processor still can't overcome the laws of physics.