I'll be very interested to see how the noise at high ISOs compares with the 20D on a pixel for pixel basis.
Large pixels are supposed to have an intrinsic advantage regarding noise levels, but this wasn't the case when the 1Ds was released shortly after the D60. Despite the 1Ds having larger pixels than the D60, noise was actually slightly greater. Of course, over all it was less, for equal size images, due to the greater number of pixels of the 1Ds, so the fact that pixel for pixel, noise was greater did not seem of much concern to anyone.
I expect a similar situation will exist when comparing noise levels of the 5D with the 20D. However, if those larger pixels are actually less noisy than the 20D pixels, it'll be a great bonus, and might be the deal clincher for me.