According to this Neowin post, not only part of the game's source code was stolen from Valve but now a beta version of the game (who knows if it's playable) has also been leaked:
Today the Half Life 2 Beta leaked across the world onto the internet via IRC channels. The release by "anon" shows it's obvious that the hackers who managed to sucessfully steal source code of the game itself also stole the game itself and who knows what else.
And with this also come the news that the game might be delayed until next year, couldn't be getting worse for Valve IMO, it's too bad really.