Someone else did a great post comparing all the available sensor sizes and came to the same conclusion - and for the life of me I can't remember who it was, but the current best body for small bird photography (involving cropping) is the 40D by a substantial margin. The 1DSMK3 with 21 million pixels still wont beat the 40D in the cropping stakes -it will be close enough to make the body very desirable, but it still comes 2nd and it's £6K!
If you're using long lenses for bird photography you need to choose your sensor size very carefully to maximise the value of the lens. I'm using the 20D most of the time for small bird shots and expect to see a marked improvement when I get the 40D.
I won't be getting rid of my 1DMK2n, it still AFs with the 2XTC where the 20D doesn't, it's a fantastic camera, and it's nice to have both formats, but for me there's little point in going any bigger in sensor size in future unless there's some drastic change in the basic technology.