This is getting tons of notice over on Battleforums, which quotes the ubiquitous Anonymous Source (he does get around), this one "from inside Blizzard," as saying that StarCraft 2's development will be "largely postponed" until Diablo III is ready to go.
Mr. (or Mrs.) Source says StarCraft 2's dev team is comprised of "a minuscule amount of people" compared to the current Diablo III team. Pretty clear which baby was gonna get its milk first all along, I'd figure. Blizzard hasn't said anything officially about the rumor. Sounds like a good rumor, doesn't really defy reasonable expectations. While it doesn't exactly fix a release date for you, it does let StarCraft players know not to hold their breath waiting.
Diablo 3 Release Date Before StarCraft 2 [Battleforums]