I've never used MF so can't really comment on this, however... Pro bird photographer Nigel Blake took a stunning barn owl shot with an 8mp 20D, it is now a 7+ foot high poster at the visitor centre at RSPB Titchwell. It looks stunning (though I've not given it close inspection), make me wonder just how big you could print from a 1Ds mk II, especially for use in large advertising posters where fine detail is not always so vital.
http://www.birdforum.net/pp_gallery/...hp?photo=40296