I spent a large portion of the last few weeks at PCF building a migration script to migrate tickets from our Trac instance to bugs in our Bugzilla instance.
I started writing SQL scripts, but then it got too hairy because there are a bunch of Trac ticket fields that have no constraints and translating them to Bugzilla equivalents required mappings and temp tables ... I abandoned that approach pretty quickly and wrote the migration script in Python.
The outcome of the migration is pretty decent. We've spent time fixing the data in Bugzilla after the migration, but I don't think there's a way to do a perfect migration because of the nature of the two bug systems.
I thought the project was interesting and mentioned it to a few people. The most common thing people respond with when hearing I was working on migrating our bug data from Trac to Bugzilla is, "What??? WHY?!?!" and their eyes would open wide with shock. I think Bugzilla has an undeserved bad rap.
The scripts are here (participatoryculture.org) if anyone else with similar plans is looking for them.
As a side note, the Python Database API specification PEP is fantastic--anyone who contributed to it should get a gold star.