Of course, as is typical with declarations like this, there is no real context for the claim. For example, it is unclear if Ballmer is referring to worldwide sales, just US domestic sales, or what. We also don't know exactly how impressive the number really is, because Windows Phone wasn't selling very well at this time last year. Remember, Windows Phone only started to make a move with the release of the Nokia Lumia 900, which wasn't until earlier this year.
Still, this does mark an impressive improvement for the Windows Phone platform, and we'll be watching to see if the momentum keeps up.