"The next time you play as Gordon will be longer than the distance between Half-Life 2 to Episode One, and Episode One to Episode Two," Lombardi told Kikizo when asked about Episode Three.
For reference, Half-Life 2 arrived in November 2004, with Episode One arriving nearly two years later in June 2006 and Episode Two following in October 2007.
However, the marketing VP offered one slight consolation, noting that the company "may [announce or show something] at the very end of the year."
Currently, Valve is putting the finishing touches on its cooperative zombie-killing shooter, Left 4 Dead, due November 18, with the studio continuing to work on updates for its class-based multiplayer shooter, Team Fortress 2.
Thanks to marmite for the heads up.