I think PGP signing could be a great way to prove authorship on publicly available messages (in those cases where rather than anonymity one does want to definitively know who wrote a message).

EDIT: If you are not already familiar with it, take a look at the PGP web of trust idea to get a better feel for the "proven authorship" use case. Some linux distros use PGP keys to prove authorship of software. http://en.wikipedia.org/wiki/Web_of_trust.