I hadn't seen this mentioned here yet, though perhaps I missed it. At CEAS last week, some researchers from Brazil presented a paper where they tracked the evolution of spamming techniques against the parallel evolution of SpamAssassin rulesets. It was heartening to see that each new SA version caught a whole bunch more spam for a while before the spammers caught on.
http://ceas.cc/2010/papers/Paper%2019.pdf -- J.D. Falk Return Path
