The only time I've seen something like that was when a system update had overwritten the nvidia version of the OpenGL libraries (Xorg has its own version which the nvidia installer replaces). I you've recently updated any Xorg packages then try re-running the nvidia installer and it should sort it out. If glxgears also fails with similar errors then that's a good indication that this is what has happened.
If glxgears is working fine, then it's probably something else. It might be that Tremulous has been built against the wrong OpenGL libraries, or possibly something else entirely, but I'm starting to get out of my depth here so I'll leave it at that rather than continue suggesting things I'm not sure of.