nVidia is going after AMD and their "Dragon" platform. To do this they are leveraging the new Intel P55 Chipset.
You see they really do not have a complete platform solution like AMD did and now they are rapidly building one.
Referred to as "Power of 3" in the PR Slides that have gotten out into the public eye, nVidia claims that P55 with built in SLI support is perfect for their new SLI + PhysX design.
They start off with a mainstream board that has 1 full x16 PCI-e 2.0 slot and a 2nd x4 electrical slot for an additional PhysX enabled GPU. The next level is the performance level. This has at least two x16 physical slots that can adjust to x8 each with SLI. The board should also have one additional x4 electrical slot for a PhysX enabled card.
The top tier is the Extreme, this will have up to 3 x16 PCI-e Slots with full x16 in each provided by nVidia chips like the BR-03 or NF200. This will allow for 3-way SLI. But the Extreme board will usually also have a fourth x4 slot for,...you guessed it; a PhysX enabled GPU.