In their presentation to the attendees at Fragapalooza they went over the technology and the fact that the SLI naming doesn't have anything to do with Vodoo's own SLI, it only shares the name "SLI". Vodoo's SLI tech stood for "Scan Line Interleave" and each GPU was responsible for drawing every other line on the screen. The nVidia technology is completely different and stands for "Scalable Link Interface" and essentially one GPU is responsible for drawing either the top or bottom half of the screen, but not quite because with the alogorithms in place to make sure each GPU's shares an identical work load it can end up with one GPU drawing the upper 1/3 of the screen consisting of a heavier graphics load while the other GPU draws the bottom 2/3 of the screen with a lower graphics load.
This is what NVIDIA themselves say about SLI:
Bus bandwidth and system communication: NVIDIA SLI technology fully exploits the increased bandwidth provided by the latest bus architecture—PCI Express—when communicating between the GPUs and the CPU. Providing 60x the bandwidth of PCI, the PCI Express interface eliminates the bottlenecks caused by previous bus architectures and allows for maximum system performance in a multi-GPU configuration.
GPU communication: Featuring an intelligent communication protocol embedded in the GPU, and a high-speed digital interface, NVIDIA SLI-based GPUs can easily communicate with one another without the overhead associated with a bus-only implementation. In addition, unique software algorithms efficiently share the workload to deliver unbelievable performance.
Image Quality: Advanced compositing, rendering, and scanout technology delivers uncompromised image quality, eliminating the “flashing” effect found in previous solutions. NVIDIA SLI multi-GPU technology also supports both analog and digital output.
The cool thing with NVIDIA's SLI is of course the fact that it is transparent to games and applications which means that any game/application should benefit from it.