Via Rob Fahrni comes this link to a J.K. Rowling interview, where show confirms that Dumbledore is dead:

(Question about upcoming deaths): You shouldn't expect Dumbledore to pull a Gandalf. I need to be more explicit: Dumbledore is definitely dead. I know there's an entire site out there called DumbledoreIsNotDead.com, and I'm sorry they're not going to like this answer.

Partisans at that last site are not convinced yet :)