Found this project at sourceforce, also reviewed recently at slashdot:

dbacl is a general purpose digramic Bayesian text classifier. It can learn text documents you provide, and then compare new input with the learned categories. It can be used for spam filtering, or within your own shell scripts. Sometimes it plays chess.

The author made an essay showing step by step how to teach a Bayesian filter to play chess against a human, on Linux, with XBoard.