MS is probably well aware of how many units they have already sent to the resellers, how many are sitting on Jumbos, and how many are sitting on ships somewhere around the seas. They also know how many they're producing each they. If there would be a serious supply problem, then denying it and even making confident claims about their expected sales would get back to them pretty hard (like, MS stock price drops down a fair bit) - and this is something I'm sure they wouldn't risk.
The only 'problem' really is the demand. Seems like people on all the internet forums seriously underestimated it... so now it becomes a tool for the MS bashing campaign? Absurd...