The one thing that I was really impressed with on a Canon 50d body, is its ability to allow custom lens auto-focus micro adjiustments for lenses that front/back focus. This is a common problem with third party lenses since their engineers reverse-engineer the lens-body communication, and thus dont have a perfect idea if this mechanism will work right.
I saw that the first lens I bought with my 50d, a Tamron 17-50mm 2.8, was back-focussing by quite a lot! In the camera body, a compensation of +17 units was needed to correct this problem (the scale for micro adjustment follows the closed integer range [-20, ..., 0, ... +20], the units are un-specified).
It took me about 1 hour to calibrate the body for this lens. I thought this was a grossly mis-calibrated lens... even though I dont know what the units of adjustments are... but I would have had to move the barrel quite a lot to correct the focus everytime on any other body that didnt have this adjustment feature.
All in all, this is a great feature on the 50d... and although it is a great to have feature, it couldnt stop me from getting the D90 instead because of brilliant colors and low noise images. Needless to say, the Tamron and the Canon 50d both went back for a refund.. and my Kata H-14 bag now hosts a shiny new D90 kit:-)