I've been working with a lowly Canon 10D for the past year or so, and I have been very impressed with the results. My main camera before that was a Pentax 67 II (MF film camera) that I bought when I got tired of lugging view cameras around (I'm older than Didger, and not as energetic any more!) I have done some comparisons between the 10D and the big Pentax a couple of times now, and the results were quite surprising, at least to me.
A year ago I photographed a scene (hand-held) at the summit of Great Blue Hill, southwest of Boston, and liked the shot, but found I could only get a decent print to 10x15". This spring I returned to the scene with my Gitzo 1228, the 10D and the Pentax, and photographed the same scene very carefully with both cameras on tripod (several shots, bracketing, to be sure). This time, to make a good comparison I shelled out big bucks for professional, hi-res scans of the Pentax transparencies (shot on Provia; the scans are 600-MB tiffs). I carefully did basic Photoshop processing of the Pentax scans and the 10D images, and then printed sections of each that would have been 16x24" prints.
What did I find? The prints are almost indistinguishable. I cannot see any more detail or any greater dynamic range with the Pentax than the 10D.
1. The 10D can make prints to 10x15" with reasonable care.
2. If you are very careful, and follow the recommended workflows of smart folks like Michael and Jonathan and a few others on this forum, you can get good prints to 12x18" with the 10D.
3. The 20D ought to give even more leeway and get more detail in prints up to (I would guess) 12x18".
4. For bigger prints you would need either more megapixels, or to try stitching, as Jonathan suggests (I plan to try that soon).
5. If you pay attention to the smart folks on this forum, your technique will certainly improve, as mine has.
I really thought my Pentax lenses (especially the 55mm WA) were really sharp -- until I did real-life comparisons with the 10D.