RP: The development cycle will be more like StarCraft 2. Wrath of the Lich King is an expansion set to a game that's very established in technology, pipeline, team, and everything else. Diablo III is a full-blown new product for us so it will be similar to StarCraft 2.
Whatever that means, heh.
Starcraft II went into development in 2003, and still does not have a release date pending. I'm guessing it comes out in time for Christmas, giving it a total development time of 5 years. If Diablo III has the same development time, that would give it a 2010 release date.
Diablo II was announced 3 years before it launched. Starcraft II was announced in 2007 and will probably come out 2 years after that announce date (according to my prediction). Thus, 2 years from Diablo III announce date also sounds reasonable.