Hey, here's an official quote about it. Apparently since Battlenet 2.0 doesn't come out till Starcraft 2, people are pretty sure that Diablo 3 HAS to come out after that.
Official Quote: 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 heard rumors of a 9-9-9 release date. Basically 666 upside down.