VR ZONE have an interview with David Kirk, Nvidia's "chief scientist" (sort of like Davros, I guess), on the NV40 and its SLI capabilities.
NVIDIA is not prepare to release any performance figures yet but two GeForce 6800 Ultra PCI-E in SLI configuration can achieve 1.87x more performance than a single GeForce 6800 Ultra PCI-E card. Certainly 3DMark2003 score will be insane and you can expect a 20,000 3dmarks score easily.
Some great screen shots of the new cards, the new motherboards that will take them, and SLI in action. More here.