There is a Vista quirk however, and Guru3d covered this in their Physx article. Here is a quote:
"Now there's a thing you will need to be aware of in the Multi GPU mode, it's actually a Vista limitation but a second monitor must be attached to enable PhysX running on the second GeForce GPU. You must extend your Windows Vista desktop onto that monitor.
To bypass that issue, most monitors have a standard VGA and a DVI connector, right? Just use both. This limitation is related to the Windows Vista display driver model (WDDM). This limitation does not exist in Windows XP. In NVIDIA's upcoming drivers, they will be offering a workaround to improve the experience for Windows Vista users.
With a single card or two cards in SLI mode you will not have this problem.
Anyway, we have inserted the two cards, then we booted up windows, installed the new GeForce Forceware 177.79 drivers, then installed the PhysX drivers 8.07.18 and rebooted.
BTW small hint to NVIDIA - I feel the PhysX options should be moved and located into the NVIDIA control panel, everything under the big umbrella is the best way to go.
In Vista just select your monitor properties / display settings then extend the monitor to the second display (make sure the faster adapter is your primary graphics card with your monitor connected to it). Now under GeForce PhysX properties we have new options available. Let's have a look at them."
Weird right? This does not occur in WinXP however.
I ran into this is Vista. I hooked up a second monitor for the time being and it worked just like they said. Nvidia is giving a workaround for this Vista WDDM limitation in upcoming drivers.
Very odd bug, definitely something to be aware as right now it seems that would preclude the possibility of an ATI GPU for 3D and an NV GPU for PhysX. It also seems like it could be a problem due to NV's propensity to release updated drivers for newer cards without also updating the .inf for older cards. So its completely possible to have a GTX 280 and 8800GT, but not have working PhysX SLI because the 8800GT isn't compatible with the 280 GTX drivers. NV really needs to disable the 2nd GPU's dependency on WDDM drivers. The PhysX driver itself is standalone, but it looks like its still recognized and bound to video card dependencies.