SDB writes a bit about using Bayesian networks to fight spam. I use the same tool for filtering my email that he does -- POPFile -- and it does work incredibly well. Furthermore, you may have noticed that I installed an anti-spam filter for the comments section here that is also built around a Bayesian network. It doesn't work perfectly yet, and some comments are hidden (not lost) due to the software mistakenly thinking they're spam, but it's getting better.
Go read more about Bayesian networks, and rejoice in one of the first truly widespread applications of artificial intelligence in everyday life.