The likelihood that CoD4 would run in native 1080p on either system is very slim. The 360 more then likely just scales it, while the ps3 probably uses its Horizontal doubling trick to achieve 1080p (960x1080)...Ninja Gaiden Sigma, for example, did this as well.
EDIT:
Come to find out, neither even support 720p. This evidence comes from the individual who busted BUNGIE's Halo 3 resolutions at 640p.
CoD4's resolution for both the 360 and ps3 is 600p (1024x600p 2xAA). Imagine that.
http://forum.beyond3...950#post1089950