I'm using Mahout, it does have Bayesian classifiers (Naive Bayes and Complementary Naive Bayes). However, Bayesian algorithms never worked well for my datasets. I went with LDA for my tasks :)

BTW, if you do want to develop a library to work with classifiers, my only "wish" would be to have a good export/import tools. Most libraries i worked with are using their own serialization formats, so it's quite difficult to import/export data without an overhead. If you write several adapters that could allow people to work with datasets through the adapters, i'm quite sure that ecosystem will get created by other people extremely fast. Some people use mongo, some use postgre, others use files, csv, serialized formats. It's great to be able to continue to use your own things without changes when running that kind of a lib.

Other "wish" thing is to have an ability to run classifier as a map/reduce job, and do it easily :)