The 12-hour maintenance will prepare the servers for the launch of the major update.
UPDATE: Blizzard has confirmed that patch 4.0.1 is going live tomorrow, Oct. 12.
Blizzard just announced that it will be performing 12 hours of extended maintenance on Tuesday, Oct. 12, which is a good indication that patch 4.0.1 will be going live tomorrow. This is the patch that will pave the way for Cataclysm and features revamped talent trees, the PvE/PvP currency changes, various UI changes, the Flexible Raid Lock System, and more.
Maintenance is scheduled to begin at 12 a.m. PDT and last until 12 p.m. PDT. What are you most looking forward to in patch 4.0.1?