Wired had a Q&A session with World of Warcraft Lead Producer J. Allen Brack at BlizzCon 08. Here's a snip about Blizzard's plans for WoW after Wrath of the Lich King:
Wired: What comes next for the game, content-wise after the 3.0 patch and after Wrath?
Brack: We've started talking about what our content plan is for post-Wrath and the first thing we’re going to work on is Ulduar Raid.
This will be a raid in the Storm Peaks, which is a Titan Zone. It will be a full 25-person/10-person raid, with I-don’t-know how many bosses, but 10, 12 bosses would not be a bad estimation. We’ll have several new quests that are going in as part of that, and then we’ll add other stuff that we haven't really figured out yet.
The whole team’s focus is on getting the 3.0 patch finished and then finishing up everything that needs to be done for Wrath of the Lich King. As soon as that’s done we’ll start talking about what goes into 3.1.