official response from Rocksteady, OP didn't provide link though
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.
The only actual feature that comes to mind would be using NVAPI with it's DX10.1-related features, which would mean they'd need to implement 10.1-path for ATI-cards to achieve same.