Fun, a number of blog plugins have picked up the hashcash ideas, e.g. http://wordpress.org/extend/plugins/wp-hashcash/.

Add 100ms-of-2011-avg-cpu computation and tie it to the submit button (avoiding any complications interleaving with user activity). So that deals with first-order dumbbots and makes life a little harder for Javscript-executing (but still volume-based) folks. Marry to a bayesian system to handle the third-order mechanical turk-style miscreants.