Some locations are only taking up to six
[Update: Amazon is now taking Xbox One pre-orders]
Right after covering the Xbox One reveal, I went out to my local GameStop to reserve the $499 system (due to the fact that I wish to cover it on launch day for you guys, not due to interest). After reaching my usual location, I found out that they were only taking a limited amount of reservations -- specifically, six. I was the sixth.
So I called around to investigate, and it seems as if many retailers in the US are only taking a few pre-orders at the moment. If you're so inclined and really want an Xbox One, you should call your local retailer and have $100 ready to put down on it.
Of course, console pre-orders are a bit of a mixed bag. No, you really don't have to pre-order to get your hands on it this November in all likelihood, but I've been doing it since the PlayStation 2, since there's nothing quite like strolling in a store, sans lines, the morning of a release and enjoying it all day long.