Joystiq has gotten their paws on, what they believe to be, a Best Buy internal memo which states Best Buy stores should expect the second shipment of Xbox 360s to arrive the weekend of December 16th - 18th, with "no fewer than 20 Pro systems going on sale Sunday the 18th" (whatever that means). Our local Fred Meyer store gave us a similar time frame for their next shipment to arrive, and our local EB (who failed at delivering us our pre-ordered system) is only guaranteeing us our console before Christmas. Point being, it looks like Microsoft will be shipping out their next round of consoles to all retailers around that time.
As an additional note, also found in the memo (which hasn't been verified), is the mention that the bonus DVD remote will no longer be coming with Xbox 360 Premium package. However, it's not clear as to whether that includes this next shipment of consoles.