Unfortunately Debian and Ubuntu use incompatible bts systems. Currently Ubuntu's launchpad based bug tracking system _bts_ is lacking accessible package version information. Bug tracking information tied to package version is essential for debian where packages go through many version iterations between releases.
Up to and including Hardy, ubuntu used apt-listbugs which referred to debian's bts with package version tracking. Even though this pulled bug information from the debian bts, it gave a reasonable indication of what packages contained significant bugs. apt-listbugs was withdrawn, because ubuntu package customization increasing has made the related debian bts irrelevant.
Topic branches and trees are ways by which package customization can be tracked.
In order to meet the Debian Collaboration Team's objective the launchpad bts must interface with the debian bts. Only this way developers benefit from the topic branches, trees of distributed package source control. To collabate bugs must be tracted across both debian and ubuntu and be accessable to both native debian and ubuntu developers.