Historically, id Software has remained extraordinarily committed to Linux. Ports of Quake 3, Quake 4, and Doom 3 are available for the free operating system, and a port of the upcoming Enemy Territory: Quake Wars is even in the works. However, it looks like id's love affair with Linux may be cooling off.
The folks at Beyond3D have spotted an article on a German website that quotes id Software CEO Todd Hollenshead as saying a Linux version of id Tech 5—the technology that will power Rage, id's next game—isn't planned.
Interestingly, Hollenshead also mentions that id Tech 5 is based on Microsoft's Direct3D application programming interface rather than OpenGL—a dramatic departure for John Carmack, who's been using OpenGL to develop id games since the original GLQuake in 1997. That said, an OpenGL flavor of id Tech 5 is apparently up and running: Beyond3D says the Mac version of Rage uses the API, and the first demo of the game was on a Mac.