Mike,
I think it depends on the type of wildlife you're shooting. There may be some benefit from the improved AF for targets that you're tracking.
The second point is the sensor resolution....2MP extra. If you're filling the frame and doing very little cropping in PP then you might see something worthwhile. If you're cropping in PP then the extra MP in the shot starts to become less and less relevent in simple pixel on target terms.
I've pondered a similar move for a while but managed to resist....the 50D (from a 30D) "ups the anti" though.
The challenges with macro are far too numerous that simply upping the resolution will give a fantastic improvement....IMO
Bob