Slightly tangential...
Getting your feet wet in the mod world can be useful. However proceed with great caution. The first rule of thumb is that things will generally take 6x longer than you expect or are told, and everyone you work with will be an emotional timebomb.
Mod work allows you some freedom to explore different areas of making a game, and is a great preparation for how people can backstab and generally screw everything up in amazingly short timeframes.
Personally I've been involved with only one mod on a major level (2+ years), however doing so I produced 6 maps, a few crummy models, loads of milkshape animations, character skins, UIs, sounds, websites, art, even PR (which I suck at)... I eventually ended up leading the mod for a good period of time. Of course it all exploded spectacularly because some members got into a fight about bugger all of nothing - which lasted the better part of 12 months.
All this teaches you a lot about managing people, which at the end of the day is what making games is about. Even as a junior programmer or an artist you still need this ability for yourself. The higher up the food chain the more important it gets.
The problem is everyone wants to be the designer, and no one wants to follow someone else's design. The biggest problems I faced while attempting to bring that mod under control were trying to standardise things and (*shock horror*) setting deadlines and stopping feature creep. Unfortunately I failed quite badly here - although in hindsight there wasn't much I could have done. This is the trouble with large teams who are basically working for their own fun and enjoyment.
Of course I myself am very guilty of exactly this - I quit a certain mod before it reached it's first alpha, because I thought it should have been more objective-based (and I wasn't very good then). Shame...
Ohh and because goose' wasn't responding to my emails either.
Only other piece of advice, if someone approaches you to work in a mod or similar project, be cautious. Ask questions. Lots. If they refuse to answer some, or simply can't, forget it. If they claim they will have something, ask them how it will be done. If someone says they will use it as a demo to get funding for a full project, run like hell. I've had this last one happen maybe 4 times, and everytime it just explodes in in a way that is both utterly depressing and utterly pathetic.
The last time this happened, someone came to me wanting asking me to work on a Quidditch (sp?) mod for a UT based game. On completion it would then land them a deal with EA, which would allow then to form a studio, which then they could license UE3 to make the full game. All within 12 months. This guy is late 30s, *really* intelligent and I respect him totally - yet couldn't see any fault in his plan.
Now. That said, you do learn from it, and that can be very useful