Hal mentions all the problems a going concern would have with an electronic banking: patent on the blind signature, RSA license also required, acceptable use policy, underlying legality.
I do think, though, that an informal digital cash system, presented as a research project or an educational game, would be able to slip between the cracks of the legal system, much as PGP has done.
I agree. An experimental money system should be fine, practically speaking. There will be no problem if no goods or services change hands. If everything is scored in points, then there is no concern about money at all. More generally, a digital money system is isomorphic to a conserved quantity server. For example, if I were to write a distributed multi-player simulation game, I could represent conserved quantities such as fuel and ammunition as the equivalent digital money tokens. That is, in order to fire, I have to "spend" a bullet. Eric