mmm you have to ask if 500mm f2.8 was in such demand why have canon/ Nikon not already created one?
good luck in selling them sigma and 11 out of 10 to Sigma for innovation in the last few years (120-300 f2.8 /300-800 f5.6 etc) but will you sell many...
One other thing, have you noticed that all of the publicity shots show the smallest camera they could find tacked on the end 350d/400d/450d when in reality the people buying the lens would use a 1d.. or d3..... which would make the lens look much smaller