Initially, Nintendo hardware always sells well. The company knows how to get the word out about new hardware and rarely has supply problems.
I think the launch sales of the Revolution will be huge, and may outsell the Xbox 360. However, long-term I think the 360 will significantly outsell it. Keep in mind I'm assuming a best case scenario in my mind for the Revolution (300-500 game library, tons of innovative games, no huge dropoffs in sales).
We'll see though. I've been surprised before, but I think the Xbox 360 will be #2.
What surprises me about this thread is even in a horrible month, the 360 was the second best selling piece of hardware. How many times did the Xbox achieve that goal? None that I can recall.