The Torque 3D game engine was already open-sourced and has been used to power Tribes 2, Frozen Synapse, and other titles.
Dusan Jocic, a Phoronix reader and developer, has been porting the game engine to Linux. He was the developer of the OpenWolf engine while lately he's been playing around with the Torque 3D engine. "I want with this engine to provide remake of Wolfenstein Enemy Territory and Tremulous and of course they will be released as FOSS and F2P."
He's been sharing progress on his independent Torque 3D port to Linux via this Phoronix Forums thread.
He's also shared this video via the Phoronix thread.
Dusan Jocic will be publishing his code soon for Torque 3D on Linux.