Reminds me of comments by Ayers and Nalebuff (you should talk to Nalebuff if you ever have the chance; I've never met Ayers) on car theft

http://islandia.law.yale.edu/ayres/forbesstopthief.PDF

They point out that car alarms and the Club simply cause a car thief to move on to the next victim but that LoJack is the reverse -- it benefits everyone even if only a small percentage use it.

Those who are really tracking down spam and fighting it benefit everyone (such as Blue Frog http://en.wikipedia.org/wiki/Blue_Frog).

You're right to be searching for answers; I think you'll find that people have them.

P.S. Have you ever seen a properly designed (http://www.templetons.com/brad/spam/challengeresponse.html) challenge response system? Just asking ;-)