According to
this article on CNN Money,
Valve is currently targeting this summer for the completion of
Half-Life 2. "Valve does not plan to reveal any additional information until the time surrounding the E3 trade show, where the game will once again be shown this year." The article also mentions that the other Source-powered game,
Vampire: The Masquerade - Bloodlines, now has a target release date of Spring 2005. Thanks:
Ve3D.
Check related article for more details.