The Canon EOS 50D, with 15 mega pixels, 1.6 crop factor, 6.2 frames per second and its clean high ISO had convinced me to retire my EOS 1D Mark 2 as my primary wildlife camera. (See 50D vs 5D vs 20D vs 1DMk2 - Bird Shootout https://pbase.com/dbehrens/50d5d20d1dm2 , which compares image quality and helped me make this decision). Also in the Diffraction Limits of 50D vs 20D https://pbase.com/dbehrens/50diffraction there are some 500mm test shots at various aperture settings to see how lens diffraction plays on the 15mp sensor vs the 8mp sensor of the 20D.
However, before I could sell the 1DMk2 I had an accident where my camera and lens fell down while mounted on tripod. The 50D shattered. Had it been a 1D my guess is the camera would have survived. So for me at least - I will stick to the 1D series for my wildlife photography.