points to a BBC article about how spam can be successfully tweaked to slip past current filtering methods
, excerpting "To finding out how to beat the filters Mr Graham-Cumming sent himself the same message 10,000 times but to each one added a fixed number of random words. When a message got through he trained an 'evil' filter that helped to tune the perfect collection of additional words."
iluvspam adds "It's an interview with POPFile author John Graham-Cumming that summarizes his talk at the recent MIT Spam Conference. You can still listen to the technical details here (choose the Afternoon 1 session, he starts about 75 minutes in)."