I sent the 40D back to KEH (see thread), and I've been checking KEH and the photo stores used gear for bodies.
There seems to be a little better selection of 50D than 40D bodies, which seems a little strange. Maybe because all the people who upgrade quickly have already sold their 40D's.
My question is whether the 50D is worth the extra money. I know the extra pixels will only be useful one time in 100. I think my eyes would like the better display on the back. As I commented before, one of the few complaints I had about the 40D was that it was a bit of a hassle to get the settings up on the big display, and I can barely make out the top display under many conditions. I also think I'd like the microfocus adjustment: I have the LensAlign gadget, and my 17-55, for example, is close enough that it isn't worth sending it in, but it would be nice to fine tune it.
I've checked the reviews, and most say, effectively, the 50D is better if you want to spend the money, but they're comparing new prices. They also like the controls on the 50D better.
It's about US$800 versus US$600 for used in excellent condition. Is the 50D worth $200 more?