Hopefully quarter 1 of next year. They probably delayed it for the big WoW expansion announcement, and get hyped generated around that. By the looks of it, it looks fairly complete, and they're probably working on competitive game play balance through testing right now. They are making WoL(Wings of Liberty, the 1st of three) noob friendlier than the last one, also making ladders for beginning players. The SP campaign will focus more on fun though, not teaching you how to play competitively (which I look forward to!)
They really can't delay it much longer, but I have no idea why they decided to make a new WoW expansion, continue SC2, AND start Diablo 3...crazy imo. Crazy in love w/money.