It looks like I may have the chance to go caving (just a tour) soon and I'd love to take my DSLR (Nikon D3100) ...however, caves are nearly 100% humidity and generally rather wet.
What kind of protection and steps do I need to take to protect my gear? If any?
Frankly, it'd be worth investing in one of those Ewa-Marine "baggies" if you're actually expecting constant wetness (they're around a hundred bucks). You can go for a stash of chammy-style wipes as well (the camera will be a lot easier to handle naked when you can), but they can only get you so far. Depending on the cave, there will be areas with almost constant dripping condensation, and it's not just water that's coming down; there is a lot of corrosive mineral content as well. It's easier to keep it out of the camera and lenses than it is to get it out after the fact. You won't need to go as far as a solid housing, but you'll probably want something studier than a sandwich bag and a UV filter (the McGuyver kit for ordinary rain).
Oh, and a general tip: there's never enough flash for what you want to do. A couple of slave units never hurt anybody.
Stan Rogers already covered the bags and cases quite well, so I'll just chime in for one little detail.
Make sure your gear is not colder than the air before you enter. If it is, water will be condensing straight from the air to all surfaces. And cars with A/C makes it really easy to artificially and accidentally "cool" your gear.
