Valve president Gabe Newell has has revealed Valve is working on a next generation successor to the Source Engine.
Rumors of a new engine have been circulating for a while, but confirmation came as Newell spoke before an audience of 4Chan's /4/ users gathered at Valve to celebrate his November 3rd birthday.

"We’ve been working on Valve’s new engine stuff for a while, we’re probably just waiting for a game to roll it out with," said Newell as recorded on Youtube.
Newell also confirmed the Source 2 engine would be completely new, instead of just another update for nearly ten year-old tech.
Source made its debut in Counter-Strike: Source and Half-Life 2 in 2004.

http://www.develop-online.net/news/4...-Source-engine