So, Scarab's finally ready for people to start looking at it. http://scarab.tigris.org/. It's got a pretty active developer community around it now. There is a prototype bugzilla -> scarab converter but there are issues with bugzilla's XML export that are being worked out. Not GNATS -> scarab yet but it probably would not be hard to write. People are now focused on porting the database support to postgres and oracle, beyond the current mysql support, which is a good sign. It's still beta but it's approaching completion quickly. BSD licensed. :)
Just thought I'd throw it out there... Brian