I wouldn't get too excited about Diablo 3 just yet. The earliest possible time it could get released is for 2010, and even that would be pushing it considerably. Basically the time line currently is:
Q4 08 = Return of the Lich King is released for WoW. The most likely timeframe for this is November considering their patch schedule at this time.
2009 = Starcraft 2 is released. This won't be released close to the Lich King expansion, since there is significant audience overlap. I wouldn't expect to see this until at least Q2/Q3 2009 which is a pretty conservative estimate. With the WoW cash cow machine going full swing, they can pretty much release it when they want to.
2010 = Diablo 3 is released. Again no need to overlap with the Starcraft 2 release.
I'll be interested to see how they go about this, the genre has been pretty much beaten to death by other games like Titan Quest and Dungeon Siege. I really wonder how much innovation they can cram in there that hasn't already been done?