Rocksteady (Batman devhouse) already made a statement about the demo:

The form of anti-aliasing implemented in Batman: Arkham Asylum uses features specific to NVIDIA cards. However, we are aware some users have managed to hack AA into the demo on ATI cards. We are speaking with ATI/AMD now to make sure it’s easier to enable some form of AA in the final game.
http://forum.beyond3d.com/showthread.php?t=54786&page=2