You know, at stock speeds the X800XT is faster for sure. But by minimal margin i think. Also, everything i have read says that the best drivers for the 6800 Ultra are not even out yet. ATI had tons of issues with drivers not giving the full power of the 9800 pro when it was released. I see this as no different. I think Nvidia is about 2 driver updates away from trimming that margin with the X800XT to nil.
As for the x800pro, i think everyone already knows how it stacks up to the 6800GT.
So, with all that said, and prices similiar enough to not effect many decisions, Nvidia wins based on availability alone.
Remember, the X800XT is technology that is at its peak, it is revamped 9800 technology. Thats why it works better "now". It is simply a "super" 9800 pro, if you want to call it that. Nvidia's 6800 series is a brand new technology, and therefore its best days are ahead of it, just as the 9700/9800 pro's were when it was in its early stages. I don't think it takes too much to realize that the technology that already existed is going to be the better performer up front. the bugs and optimizations have already been worked out. Give the 6800U and GT about 2 or 3 months for drivers that better utilize its power, then we'll see where the performance stands.
BTW, ATI is about 1 more paper launch away from losing even more ground in the GPU market. When someone goes to newegg and has 20-30 ultras and GT's to pick through, and "1" OEM powercolor X800XT, it is not hard to see why so many are choosing Nvidia this go-around.