Today we released our first version of a bridge for punbb forum and Mantis issue tracking software. Both are GPL and so naturally is this bridge. It allows the use of a single sign on to both systems, and password changes are synced.
UPDATE June 2010: This bridge is no longer maintained, as I have long stopped using punBB. Now that its successor fluxBB has launched a new version, I may be tempted to reexamine the code in the future.