My introduction to hashcash, which is interesting. RPOWs are (briefly) digitally signed, unique, audit-tracable bits of hashcash. Verification and the all the sensitive stuff is done inside of an IBM 4758 crytoprocessor, which is (a) expensive, (b) very, very secure.
The spread of Guerrilla cybercash is one of the things I'm predicting will result from the dissemination of TCPA-ish technologies, as I outline briefly here. Hal Finney, being the legend that he is, has already done cybercash for the nearest thing we have to a consumer-level TCB.
One thing he alluded to in his presentation is that he's taking advantage of the 'remote attestation' features of the 4758, which in other contexts (such as DRM systems) would be seen as a bad thing. Remote attestation is unique to the 4758 right now, but if it becomes more widespread via NGSCB and TCPA, then things could get interesting.