Beyond 3D forums, the PS3 version doesn't run at 1080p. Heck, it doesn't run at 720p. Rather, it runs at 640p -- still enough to be "HD," but far from the "full HD" experience Sony usually touts.
Pixel hunters were enraged when it was discovered Halo 3 ran at a lower resolution on Xbox 360 than advertised. However, should people stay away from the PS3 version? Probably not. Many Beyond 3D users note that in spite of the lesser resolution, the superior effects of the PS3 version gives it the visual edge: "the PS3 version looks nicer IMO. Everything just blends in nicely and makes a seamless contrast between front objects and background objects," noted forum user Nebula.
[Update: PS3 runs at 640p. Xbox 360 does run at a higher resolution: 720p.]