There will be downtime tomorrow for the U.S. realms between 5:00 a.m. PST and 11:00 a.m. PST. The maintenance will undoubtedly be getting the servers ready for the release of Wrath of the Lich King Wednesday night.
We're hopeful that this will be the only downtime this week. When Burning Crusade was launched the servers were only taken down the a couple days prior to its release. We expect the same thing to happen for Wrath.
There is also no indication that a patch will be delivered with this downtime. We give it a 50/50 chance of a small patch coming out the night of Wrath's release however, and wouldn't be surprised at all to see some hotfixes go in place over the next several weeks.
Of course, this downtime could be setting up another stage of the world event happening tonight.