I think we all assumed that World of Warcraft held up development of StarCraft II. But in a recent article from Eurogamer, Blizzard's Vice President of Game Design confirmed that the work on World of Warcraft consumed the time and resources of StarCraft II.
I was surprised to read that there has been a playable version of StarCraft II for four years now. I did see a playable version at GenCon 2007, so it's not that much of a total mindblow. But they've been sitting on it for a while, taking their sweet time.
You can hardly blame Blizzard for it, though. They make upwards of a hundred and fifty million (or more) dollars every month on World of Warcraft just in subscription fees, not counting the games that are purchased and registered. I wonder why Blizzard bothers making any other games, let alone two sequels.