According to a supposedly leaked Best Buy internal memo, each store has a mandatory 25-unit quota they need to save for 12/31. Anything excess units can be sold whenever, but the local circulars will advertise each Best Buy having 25 units in stock for people to buy on New Year's Eve.
We don't have confirmation of this, but we've seen them do this kind of thing for Wiis and Xbox 360s, so a big Sunday Sale isn't out of the question.