Microsoft has published, what I guess is the last press-release before the Xbox 360 launch tonight in the United States.

Microsoft says 4,500 retailers will be holding midnight madness events throughout the States. At 12:01am on November 22nd the first die-hard Xbox 360 fans can get their next generation Xbox. Xbox 360 Zero Hour event guests in the Mojave desert can buy one this evening already.
Microsoft did not make any more comments in this press-release about availability of the Xbox 360 consoles. As reported, there are rumors of very low stock of Xbox 360 consoles on each of the retailers opening tonight.
We will know for sure tomorrow, if the Xbox 360 is a sell-out or not.