Nvidia has released a small PhysX patch for Batman: Arkham Asylum title. Description says:

In order to experience Batman: Arkham Asylum to its fullest extent when running PhysX on GeForce GTX 480 and 470 GPUs, users will need to replace the existing RRB.DLL file with a new one. In most configurations, the RRB.DLL can be found in the “\Square Enix\Batman Arkham Asylum GOTY\Binaries” directory.

If it is not located in this directory, please do a search for the filename on your system.

RRB.DLL is responsible for GPU accelerated Rigid Bodies calculations – large-scale physics effect, mostly memorable from Scarecrow nightmares.

You can download PhysX Rigid Bodies patch from here (2.8 mb)