Source: Planet Half-Life

We were going to hold off on reporting on this until there was official word from Valve, but now Slashdot is reporting (by way of GamersWithJobs) that at least some of the Half-Life 2 source code has leaked.
Later on it was confirmed to be real

Gabe Newell confirmed it later saying that they had noticed suspicous activity recently...
1) Starting around 9/11 of this year, someone other than me was accessing my email account. This has been determined by looking at traffic on our email server versus my travel schedule.
2) Shortly afterwards my machine started acting weird (right-clicking on executables would crash explorer). I was unable to find a virus or trojan on my machine, I reformatted my hard drive, and reinstalled.
3) For the next week, there appears to have been suspicious activity on my webmail account.
4) Around 9/19 someone made a copy of the HL-2 source tree.
5) At some point, keystroke recorders got installed on several machines at Valve. Our speculation is that these were done via a buffer overflow in Outlook's preview pane. This recorder is apparently a customized version of RemoteAnywhere created to infect Valve (at least it hasn't been seen anywhere else, and isn't detected by normal virus scanning tools).
6) Periodically for the last year we've been the subject of a variety of denial of service attacks targetted at our webservers and at Steam. We don't know if these are related or independent.
This really sucks for Valve. This was their big release, and now it's screwed because of some hacker. I really hope this guy gets caught. Valve deserves to earn money for their 5 years of hard work on this game.