The closest to an answer to this question came in the from of a blue post from a Bliz community manager on the message boards:

Today we announced that StarCraft II: Wings of Liberty will not be ready for release by the end of 2009. Over the past couple of weeks, it has become clear that it will take longer than expected to prepare the new Battle.net for the launch of the game. The upgraded Battle.net is an integral part of the StarCraft II experience and will be an essential part of all of our games moving forward. This extra development time will be critical to help us realize our vision for the service.

Our mission is to develop games that deliver an epic entertainment experience. As part of that commitment, we will always take as much time as needed to ensure that our games and services meet the expectations of our players and the standard for quality that we set for ourselves as developers. As we work to make Battle.net the premier online gaming destination, we’ll also continue to polish and refine StarCraft II, and we look forward to delivering a real-time strategy gaming experience worthy of the series’ legacy in the first half of 2010.

We will share more details about the beta, Battle.net, and StarCraft II’s launch in the future. Keep an eye on the official website at http://www.starcraft2.com for updates, and as always, thank you for your continued enthusiasm and support for StarCraft II and all of Blizzard Entertainment’s games.

So "first half of 2010". Popular speculations suspects that the delay is more waiting for BattleNet infrastructure than SC2 itself.