Update on 2002: Intel Labs (Russia) has started implementing an open-source C++ library called PNL (probabilistic networks library), which is closely modelled on, but independent of, BNT. A beta release should be available by December 2003. Click here for a photo of the team (taken April 2002).
Update 2003. Lauritzen et al. have started a project called gR to implement an open source graphical models library in R.
Update 2005 Intel released PNL on sourceforge.