Those of you who are interested to see just how fast two Geforce 6800 ULTRA's together can go should check out the following article at Anandtech.

If NVIDIA is able to get their SLI certification program successful enough and if motherboard manufacturers are able to get SLI boards cheap enough, then the upgrade value of SLI is significant. We've already seen that going from a single $200 GeForce 6600GT to a pair of them offers performance greater than that of a single $400 GeForce 6800GT. Take into account that the price of these cards goes down over time and you're looking at a pretty decent upgrade path for the future, requiring minimal investment today.

Personally I think that's the true potential of SLI, upgrade path, it's good to be able to keep the same PC for a few years and not get totally out of date, SLI could very well makes this a reality.