Now that I have thought about it some more its very possible that Microsoft has better fish to fry than FFVII remake on the Xbox 360. As in a cost/benefit analysis they could probably spend their money better elsewhere.
Personally I don't see it being released until after 2011/2 so the question of which systems they might put it on have to include the possibility of a next generation system. In that case its not a given that the PS3 version will be the best and its not a given that the PS3 version will be the only version.
What system can do justice to the elevated expectations of what a FFVII remake ought to be? None of the current generation systems! Why compromise on resolution, frame rate and rendering detail? The PS3 is probably too weak for such a massive game so for that reason alone one has to consider the next generation. SE will not rest until Cloud's earlobes have more polygons than Nathan Drake, buns included.