The 7D now brings a further 3 million pixels to the table which isn't insignificant. It wouldn't have turned my Kingfisher shot into a masterpiece, but it would have added welcome resolution to that final cropped image had I had it at the time.
These crops show the the maximum subject size I could have recorded that Kingfisher at (full 1:1 file size) from each of the following cameras. Obviously, they're not 100% crops, but they are to scale.
If you're into wildlife photography, you really need to get a grasp of what these sensor sizes and pixel densities mean before you go and spend a whole load of wonga on your next camera.
Of course, the closer you can get to your subject and the more you can increase his size in the viewfinder, the less these differences matter, but the fact is you just can't get that close, not with any regularity, and the more reach you have the better in most situations.
EDIT.
I've been asked to add the 1DSMK3 and 5DMK2 into the above comparison chart. There are actually slight differences in sensor size between the two but so slight as to make little difference.
