The in-game breaking news text just announced extended maintenance for tomorrow. This again, is a strong indicator that patch 2.4 is on the immediate queue. We can expect the down time to be at least until 11 a.m. PDT. This normally is substantially higher on patch days, usually lasting until around 1 p.m. or 2 p.m. PDT.
WoW Insider staff will be burning the midnight oil to make sure that you have a good list of patch mirrors and addon information ready to go when you wake up tomorrow morning.
Additionally, Belfaire has said that the patch is more around 300 megs, not the 900 megs that is showing up on Gamespot. Further more, the Gamespot download is a 2.0 to 2.4 update, which very few people will need tomorrow.
Updated 12:20 a.m. PDT: Belfaire is having fun with me. First he confirms, then he edits, then he edits again...
Updated 1:12 a.m. PDT: Updated Gameespot info, thanks to those who left comments on tonight's posts. And a fix for Belfaire. =)