You won't see a pro body with such high pixel density - for a while at least. the 1Diii has only lousy 5MP in the same sensor area.
And 18MP has a big advantage over 10MP period, so even if you shoot your bird in a cage, the 7D still gets better detail.
The 7D is also equipped with high def video... tired of still shots? grab a few minutes of Planet Earth style video while you are at it.
The 7D also has newer gen of AF system, chances are canon will dump the AF system in 1D3 altogether before coming up with an ultimate fix. So get rid of 1D3 while you can still sell it for a few grand and get a 7D and good glass. it will be a lot of work to convince your buyer your 1d3 is 'flawless' once the 1DIV is out.
And of course the 1.3x is an oddball by itself. You either need reach (1.6x) or coverage (FF), the 1.3x is nothing but an overpriced compromise.
Max
Would there be any legit reason to drop a MKIII for the new 7D when used for BIF apart from the crop factor and the possibility that AF may be better?
Fairly broad question but I'd love to hear other opinions.
Richard