I tried the 50D today in jessops! Looks great, feels very sturdy (not any more so than the 40D though) and operated in very much the same way. The autofocus to me seemed identical to the 40D (single shot mode, using one focus point) The biggest change regarding operation or usability is definately the screen which looks really nice. In menus it isn't really noticeably different, but viewing photos is a different matter, lines are much sharper and more defined, it's easier to see the focus and such...it looks beautiful in live view too. It looks like a print, but lit up, rather than a screen. :P
Anyway, so I did some shots of the same scene using the same settings (I shot in jpeg and raw) but for the moment I only have iso3200 side by side with the 40D using standart picture styles and AWB, but the same exposures. I'll try to get some more up shortly, if people are interested.
There is the full (uninteresting) scene. :P As taken with the 40D
Here are 100% crops with the 50D on the left and 40D on the right, you can see that the 50D shot is closer in due to the extra resolution.
I focussed both on the words "great savings" using magnified manual focus liveview. The 50D image looks soft to me - but I'm positive the focus must be slightly offor at least different. :/
NR was turned off for both.
(both were taken with the 17-85mm lens, and it was resting on a shelf, I did my best to replicate the scene with the other camera.