European WoW site, which as usual, has better info than its U.S. counterpart. The service announcement reads "Weekly Maintenance - 26/03. This Wednesday there will be an extended maintenance to apply patch 2.4.0. Please note that all European realms will be offline from 03:00 until 11:00, Paris time (CET)." March 26th over in Europe is March 25th here in the States.
All we're waiting for is the U.S. version of this announcement, which will soon come. After that the final patch notes will drop, which we'll have mirrored here for you as fast as we can get them up. As the posts below this one indicate, down time started about 30 minutes ago and is expect to last until at least 11 a.m. PDT.
Happy patch 2.4 day! WoW Insider will be updating frequently, stay tuned.
Updated 7:06 a.m. EDT: Belfaire has responded again on one of those "is 2.4 today?" threads: "Enjoy the new patch, folks!" This serves as confirmation for the U.S. realms. And you have a tag this time Belfaire. ;-)