The announcement of Starcraft 2 got one monkey off Blizzard's back. The other three (Diablo 3, Warcraft 4 and, for me, Lost Vikings 3D), they're working on, one at a time. First up is another Warcraft RTS, which Blizzard's Rob Pardo says is definitely "on the table".
The way that we develop things, each time a development team becomes available, because they're done with creating the games that they've been doing, they have a lot of say over what their next game is. So what it would take is for one of our teams to decide that they want to make Warcraft RTS and of course, there'll be some talk about doing that and I'm sure after StarCraft 2 ships, it will be one the ideas on the table for that team to do, but that's pretty far off in the future and it would really be determined by a combination of the dev team and the company leadership.
This "Warcraft" name, it's pretty big these days. Think an RTS might be a no-brainer move for them, since you could release a Warcraft Activity Centre on the V.Smile and it'd still shift 20 million units. Blizzard's Pardo Hints at Warcraft RTS; Not Interested in Consoles [GameDaily]