I wrestled with choosing between a x800xt PE or 6800U for the longest time. I ultimately went with the x800xt because:
1. DVD playback has always been better with the ATI based cards and I use my PC for movies more than games these days.
2. The ATI cooler doesn't generate as much noise.
3. The power source issue. I know nVidia has lowered the requirements, but I'm still not comfortable with it and because it requires two molex connections.
4. I'm not entirely convinced that I'll see the benefits of SM 3.0 to any appreciable degree within the next year or so. OpenGL is still better with the nVidia, but the ATI is good enough for my tastes. Supposedly ATI is doing a rewrite of the OpenGL driver from the ground up anyway, so I might see some advances in this area in upcoming driver releases.
5. Availability. The above four reasons are relatively minor. I still could have gone either way. Whether or not I could find either card was the biggest issue. Ultimately, it came down to which ever card I could find first. As it turns out, I was able to pick up my retail x800xt PE last month via special order from CompUSA. I've still yet to see any place other than eBay where I can get a 6800U.
And no, I'm not interested in either the 6800GT or x800Pro.