LinuxGames relates an exchange a reader of theirs had with id Software Technical Director John Carmack over plans for Linux ports of future id games (thanks Ant). Linux Wolfenstein is "unlikely," as it is being developed by Raven, and though id has been a longtime supporter of Linux, there are "no firm plans" for Linux RAGE or DOOM 4, "but it certainly isn't off the table." The reason for the uncertainty isn't that his code is less portable, but that OpenGL support is becoming less practical:

Wolfenstein is pretty unlikely, since it was developed at Raven, and published by Activision. There are no firm plans for linux ports of the idTech 5 titles, but it certainly isn’t off the table. I don’t think it will be very difficult to get them running on the binary nvidia drivers, but bringing them up to functionality and acceptable performance on other OpenGL drivers would probably be a more significant undertaking than we could afford.