Bit Tech heard from NVIDIA that the GPU PhysX pack will be available as soon as next week. NVIDIA will release PhysX driver updates nearly every month and it will be available to all owners of GeForce 8, 9 and GTX 200 series graphics cards.
The current driver will work with only one GPU currently, although that includes iterations of SLI since the driver sees multiple cards as one. In this respect it will automatically balance graphical and physics data depending on what the game requires - this will hinder your frame rate a little, but it should make other factors look and feel a little more realistic (NB: "should").
Future driver releases will allow assigning of PhysX to a separate GPU - so if you upgrade from a GeForce 8800 GT to GTX 260 for example, the GT can work on PhysX data only. We double checked and you do not need an SLI board to do this - just one with multiple PCI-Express slots - great news for those with X38, P45 or even P35 boards - we can't imagine PhysX needs that much bandwidth if previous hardware used a PCI interface!