Andy, Piet, David Walsh's solution works for him because it is a "club" solution. This article from 2002 is worth a read:
http://web.archive.org/web/20110607161529/http://diveintomark.org/archives/2002/10/29/club_vs_lojack_solutions

Basically, such a honeypot concept only works because not everybody has it, and it's not worth the spammers time to work around your "fix". Quite simply, they're trying to spam thousands of sites, not just yours. If such a solution was to become commonplace, then it would become worth their time to work around it (and working around it is trivial), and suddenly it would not work anymore.

Any solution to "spam" has to account for everybody. You can solve it for yourself, but that does not scale. Scaling a spam prevention mechanism requires a lot more work than something quite as simple as that.