I've experimented with a small helmet mounted camera, but found quite a few drawbacks when used underground. The main problem I found is having to review what you've just filmed, to make sure - for example, that the lens hasn't fogged/got a drop of water or mud on it. Imagine filming all day, downloading the card and finding it's all out of focus!
In the end decided it is now easier to just carry a camcorder in a waterproof housing, much bulkier, but you can see what you're getting.
Might be good though for lowering down big/unstable shafts, strapped to a torch, to see whats down there?