A recent rumor that Diablo 3 will be released BEFORE Starcraft 2 actually prompted Blizzard to officially address the issue of the release date.
So Diablo 3 will come out AFTER that … possibly by the end of 2009. An encouraging news will be if Blizzard have a playable Diablo 3 demo at Blizzcon.
Update: Here’s the quote from Rob Pardo:
A lot of the way we develop is: We wanna make sure the game is really fun and the visual fidelity we want … we wanna make sure we have a small area that feels really done so we know what to build. So even though it looks really complete a lot of the content is just not there. We have a long ways to go to build each of the different acts in the game and put in all the quests and all the different monsters. We have a lot of development ahead of us before it will be something … It will definitely be out after StarCraft II.”
I hope this answers the last comment!