Jeremy Bowers has created (but not released) a spam editor that tries to defeat Bayes filtering.
Writing the program was easy, walking through the spam and changing all the key spam words into something else took me many hours. ... The point of this work is not to help spammers, but to deflate some hopeless optimism in the community about how good bayes filtering is going to work.
Before you give up on Bayes filters, note that it took him "many hours" to rewrite one Nigerian spam to evade an average filter. He thinks spammers will inevitably defeat these filters, as he explains in detail. Even if he's right, I would prefer more evolved spam -- receiving blatantly obvious junk like "See Teen Farm Girls Have Sex with their Animals Free" makes me feel stupid for not managing to filter it.