Diablo III was announced over a year after Star Craft II so one would expect it to be released after SCII. Apparently not, though, according to the industry's most famous rumor-monger, Anonymous Source.

An unnamed source, no doubt clad in a trenchcoat and sunglasses, told Battleforums that "[StarCraft II] is still being worked on, but only by a minuscule amount of people compared to what [Blizzard] put on the Diablo 3 team.". In other words, they'd be postponing SCII on purpose until D3 is good to go. For what reason? Blizzard's proven they know how to work an adding machine so if they're really doing this, they probably have a good reason for it.

As you'd expect, there was no confirmation of any of this by Blizzard so it could be bogus. For all we know, they might be pulling people off of SCII because of how close to completion it is. Or so we'd like to think. No release dates were set for either game so we might never know the difference.