Some XBox 360 games which I heard aren't really HD (just lower res rendering upscaling towards HD, like for instance PS2 games or DVDs being played on the PS3):
Tony Hawk's Project 8 1040x584
PGR 3 and Tomb Raider 1024x600
Perfect Dark (like Halo 3, 193,280 pixels less than HD) 1138x640
Anyhow many people won't notice as many (70%) XBox 360 owners in the US (Halo 3's main market) according to NPD research are unaware of high definition graphics.