reportedly official response from Rocksteady, OP didn't provide link though
Quote:
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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.
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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.