From what I read the 360 is not truly 1080p capable on with upscaling. Also the resolution on the back of the box is what resolutions it can support not the native res. For example most of my game say 720p/1080i/1080p on the back, this is just the resolutions they can support.
From what I understand most of the 360/PS3 games aren't 1080p but are upscaled to 1080p. GTA4, MW2 are both 640p native and are upscaled to 1080p on both 360 and PS3. Although most games are 720p, it just depends on your TV if the game will upscale to 1080p.