Empire offers an interview with Blizzard's Itzik Ben Bassat on the topic of World of Warcraft (thanks Kotaku). The final answer offers the obvious point that with over 8 million WoW subscribers, they are not looking to create the same game again in a remark that does indicate plans to further explore the MMORPG market in the future:

When we announce our next MMORPG it’s not going to be another WoW - we’re not a company that tends to tread the same ground. It’ll be something innovative and new that really brings entertainment to another level.