I think there's a lot than can be done in this field - not with micropayments, which everybody seems to think about first when they see this. What I find a lot more interesting is the possibility of many competing Internet currencies not tied to any national one.
How about turning HN karma into a currency? Probably no mass-market appeal... what's lacking is a killer application for digital money and when it comes, it'll probably be in a form that doesn't seem like money to us now.