As with everything related to the perennial whisper-fest that is Half-Life's release schedule, take the following with a medically appropriate amount of salt. About a kilo, I'd say.
[Update] Yep, it's bollocks.
Doug Lombardi issued a statement to Game Informer on this subject which seems pretty conclusive:
"This is fiction (aside from the fact that Mike D is a real person)."
So that's that. You can see the tissue of lies below.
A user going by the handle of ryuuk made a post on the Steam forums - screengrabbed by Half-life fan site Lambda Generation - claiming to have had access to former Valve programmer Mike Dussault's resumé through his work place. Dussault left Valve last year after a decade's service.
"According to [Dussault's] resume, Episode Three was put on hold when Valve decided to move away from the episodic model in late 2007, right after Episode Two’s release," ryuuk claimed.
"He was a techinal advisor on a cancelled Half-Life title, made outside of the company. Dussault’s work on Half-Life 3′s world programming, and the scripting system (between 2008 and 2010) is also mentioned in the resume."
The post was almost immediately deleted, and ryuuk was slapped with a perma-ban, an action which has only heightened belief among conspiracy-theory lovers, as moderators usually let such threads stand. If the leak is genuine, ryuuk may well face consequences for breach of privacy and confidentiality if identified.
Rumours of an externally-developed Half-life episode surfaced last month, when it was claimed information harvested from the Deus Ex website hack included a CV outing Arkane Studios' involvement. Concept art has been linked to this alleged project, which is believed to have been either Episode Three or Four.
Valve has repeatedly confirmed it is still actively working on the Half-Life series, but has gone quiet on the subject of what form the next instalment will take - and when it will surface.