The guy who broke into Valve Softwares computers and stole the source code has made a short speech on some IRC channel. From Half-Life 2 Online Source:

To all those saying HL2 was delayed because of the hacking action, ill just remind you that what was leaked is what Valve has, nothing more and nothing less, so its pretty good to understand it wasnt finished yet. 
Also I'd like to point out the E3 [demo] was one big fake by Valve. Ever tried to kill that soldier in physicstown ? Notice the door will still get kicked ? No scripted sequences ? 

To everyone saying this is no Beta: I'd like to point out that THIS is what you wanted Valve to release on 9/30/03, theres no such thing as a much better release in Valves network! 

To Valve: I suggest you stop lying to your customers about how much was stolen/compromised, or I'll have to release everything just to prove my point. And you know what you got, as I do.