Captcha: Telling humans apart from machines
"Captcha":http://www.captcha.net/. Bill Gate mentioned this in his Davos talk, but the idea is that if you don't want spam send something back which computational hard for a machine, but easy for a person. Here is the CMU version of this.
Ticketmaster uses this as does quite a few bulletin boards and some other places. He's right I think that its the answer long term to spam. The other piece you need is unforgeable email so that you actually know if it says, I'm from somewhere I really am.