As others have mentioned, you can checkout packages with
bzr branch lp:ubuntu/PACKAGENAME for the current developmental release or
lp:ubuntu/lucid/PACKAGENAME, if you are looking for the source from Lucid for instance. Source packages from Debian are also imported. These can be fetched with
This also allows us to easily browser the source of any Ubuntu or Debian package on-line via bazzar.launchpad.net. For instance, here is the banshee source package:
The vast majority of packages in Ubuntu are availiable as bzr branches. As of this writting, only 597 of the thousand in Ubuntu are currently not up-to-date (This can be tracked on http://package-import.ubuntu.com/status/).
One of the major bennifits of distributed version control is that it can help with merging. Tasks such as viewing history and annotating to find who made a specific change and why are also made easier.
A good introduction to the Ubuntu Distributed Development project can be found here:
Documentation and how-tos can be found here: