Same here. We're using a tool called FogBugz at work. It would be well suited for such a project, but unfortunely it's commercial software. It even integrates with subversion and cvs, which makes it easy to track the actual check-ins related to a bug report. IIRC Mantis bugtracker has most of these features, so perhaps we should utilize the existing bugtracker? It doesn't seem to be used much.
Oh yes, FogBugz rulez. But it's not totally commercial, you can use it when you don't need more than 2 accounts for free, maybe it could be still usable for us. I don't know about Mantis, I'll take a look.