Tuesday's are usually patch day, and both World of Warcraft and EverQuest 2 are pushing out patches today. The World of Warcraft patch, 4.0.1, will require 12 hours of downtime and is so big Blizzard pointed out the instructions to use the background downloader. I couldn't find patch notes, but I did find a couple of forum posts about class mechanics changes, changes to glyphs and a cap on the number of members in guilds. The last will affect not only The Instance podcast's AIE guild, but also an organization familiar to Eve players, the Mal'Ganis server's Goon Squad.
UPDATE: WoW Insider has a post on the patch.
Meanwhile, SOE's EQ2's GU58, Arcanum Revealed, is expected to only require two hours of downtime. Feldon has posted the patch notes on EQ2 Wire already.