Some tests have the 6600gt on top, others the 6800nu. It's safe to say that in general, at average settings, the two cards are "pretty close". The 6800nu does start to pull ahead at higher resolutions and settings, but at that point, you're approaching unplayable frame rates anyway, so it's not a huge deal.
However, the 6800nu does have some important advantages over the 6600GT for the enthusiast. Using RivaTuner, additional vertex and pixel pipes can potentially be unlocked. If the user is lucky, 16/6 is possible, which is what both the 6800GT and 6800U use (although at higher clock speeds than the 6800 is capable of). With pipes unlocked, the 6800nu enjoys a much larger performance lead over the 6600GT.
And while I haven't seen many reviews on it, the 6800nu seems to have a greater overclocking potential than the 6600GT.
We haven't heard a whole lot about the 6600GT because a) they are yet to hit the market in big numbers from all manufacturers, b) they are in many cases significantly over the $199 MSRP, and c) there have been several excellent deals on the 6800nu in the last few weeks, making the 6800nu a better buy.
Once the 6600GT AGP is in full force at $199 and the deals on the 6800nu end, there will likely be a lot of buzz about the 6600GT.