Yeah I don't think they make a linux version of the game.
That said, it is still possible to run the game on a linux machine depending upon what software you get a hold of and all that stuff. Looks like this guy got it to work with Debian.
http://www.linuxquestions.org/questions/showthread.php?t=331293)
I also came across this little tirade by some dude named "ModernRonin."
> Are you some Linux loser who claims everything is better on Linux than on Windows?
Nope, only the things that really are. And it appears that stuff based on the Q3:Arena engine (JKII, RtCW, etc) run better under Linux that Doze. Ya got a problem with that? Fine, go argue with the benchmarks. They won't care.
http://www.mrcranky.com/movies/spirit/34/6.html)
Wierd that it was on mrcranky.com....looks like some kind of store. It is possible to play JK2 on linux, whether you can get everything to work depends on your distribution and your savvy.
I couldn't really find anything on Ubuntu doing a quick search... just look around. : )