Last I looked, very roughly, Wii was ~8.5m ahead of 360 lifetime in USA.
Normally this might be insurmountable but, with Wii2 coming in 2012, it seems possibly, even likely, for 360 to end up outselling Wii in USA.
More than likely Wii2 will mean the end of Wii. Sure it will continue selling some, but all Nintendo's efforts, advertising, and games will move to Wii2, and Wii will likely die out quite quickly. Stores only have so much shelf space as well. Not too mention, Wii sales are falling quite handily right now without any help (though the rumored drop to 150 may help a bit).
For example, 360 can easily sell 6-8 million a year in USA. So that 8.5m could almost be erased in one year if the Wii becomes a non factor. Latest rumors place the Wii2 of course in 2012, and the next Xbox in 2014. So there's also plenty of time. Not even to mention, as a hardcore console, I expect X360 to have much longer legs once it's succesor hits the market than Wii (much like PS2 maintained legs after PS3)
Of course things can go wrong with this possibility, but as i say, it seems possible, if not likely, that 360 could end up the best selling console of this generation in the USA.