A forum post on the Enemy Territory: QUAKE Wars Website by id Software's Timothee Besset outlines the game plan for the Linux client for Enemy Territory: QUAKE Wars (thanks Ant and LinuxGames). While a penguin-powered client is indeed in the cards, it does not sound like it will be in the retail box:

I don't want to commit to any schedule for the ETQW Linux client at this point. Most of my time is spent working on the game itself, on the Linux dedicated server and on the network infrastructure.

Once those things wind down (in the upcoming weeks) I will start working specifically on the client. But fear not, ATI and NVidia have expressed interest in getting a Linux client early, so they can review how it performs and get their drivers adjusted. When we release, we'll release a high quality product, just like we do on Windows.

In any case, don't expect to find Linux binaries on the retail DVD. It's hard enough for everyone to get gold masters done, there are enough things to check and worry about, I will not place additional strain on the procedure by trying to squeeze my additional set of files on there.