The link below is to a page that extensively looks at a series of experiements and tweaks to bayesian filtering methodology and training methods for one popular bayesian filtering program called BogoFilter. The point is not BogoFilter but the interesting range of experiments and analysis that has been done. It's worth noting that some of the items -- for example, the header field tagging -- have already been implemented in PM's bayesian filters.
There's some pretty geeky stuff here, but interesting none-the-less.
http://www.bgl.nu/bogofilter/