MGS 1&2 were ported to every system known to man. MGS3 was the only Sony exclusive.
I'd consider this to be a timed exclusive at best. Its coming out for the PS3 in less than 6 months and there has been no announcement about even development starting for other consoles.
Porting to the 360 would take another year at least, so this will at worst be a timed exclusive. This "news" is brought up every week it seems, nothing changes.