I've not managed to read ALL the posts here yet, so it may be that someone's already mentioed this...
Could this not be handled in much the same way as the metal detectoring thingy? I remember that there was a bit of a flame war with them a short while back and it culminated in an ethical code of practice being worked out.
http://www.heritageaction.org/?page=heritagealerts_metaldetecting
Playing devil's advocate here, I think that geocaching COULD be a great way to get the young 'uns out into the countryside and learn a bit about their history and surroundings. If done responsibly.
Can "we" work with "them", perhaps, so that they can continue to enjoy their hobby, us ours, and nothing and nobody gets hurt?
I know there'll still be gangs of renegade geocachers around who insist on kicking down bus stops and putting small tupperware containers where they shouldn't ;o) but maybe awareness is the key - maybe they don't realise they're causing damage...
I feel I should run away and hide now... ;o)
G x