One of these days when some of my other projects calm down, I'd really like to get involved with nexuiz development. How would I go about learning how various parts of the DP engine work, creating content for it, and other things? Maybe I'm being a bit over-ambitious here (I've never gotten down and dirty with a "real" programming language like C), but I'd really like to be one of the people to help bring DP/Nexuiz up to commercial standards one day.

The best advice I can give about learning programming languages is google.
Really, I see how students here at uni fail at learning the basics of java, and others who simply use google to find tutorials succeed without problems.
Google for C tutorials and learn the basics of C.

And about the DP engine, can tell you how I did it: simply read through the source and see how it works.
Think about a specific change you want to make, look through the filenames, think about which file could contain the necessary code, sooner or later you'll find it.
This is how it works for me, I want to do certain changes, and simply try out things. That's - for me at least - the best way to get to know another source code.

and join #darkplaces on irc if you have questions about the engine.

well, I do know some real simple C, and I have programmed before so it's not a new concept to me (in other words once I pick up on syntax I'm good to go). I did attempt to make a mod some time ago, and I did make some progress but I was too involved with other projects at the time, and it slipped through the cracks.

When I feel I'm ready to contribute something I'll ask for a subversion account, but right now I've got my hands full with other projects.