Gaming Illustrated have a posted an Interview with Blizzard Vice President Bill Roper about WarCraft III: The Frozen Throne, the expansion pack for WarCraft III:
G.I. : Is it possible, with everything you've created for this expansion pack, that The Frozen Throne won't be the final Warcraft 3 expansion pack?
Bill Roper: We never close off the possibility, but for us whenever we do an expansion I like to think that we almost over delivery from what everyone expects from us. But it's really important for us to truly make an expansion have a lot of meaning. We don't just want to put three scenarios and a new map on a disc and called it an expansion set. We want to bring something new and something different into the game and the Warcraft world. We haven't talked about doing another expansion pack.
Traditionally we've done an expansion pack like crazy, then moved on because we finally exhaust all the ideas we could get into WarCraft 3 (in this example), and the development team has done everything they can to cram every last bit of gaming goodness into it. I really don't know, we don't have plans right now to do another expansion, but I'll never say never. It depends on if there's a brainstorm that came up to do something new and different that we've not done already.