No one knows if this is real or not.
According to several Half-life related websites and HL related IRC chatrooms word is the thief who stole the Half-Life 2 source code from Valve is threatening to release the entire source code. Here is his/her statement:
"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."