It's no secret that Canonical is a large proponent of Bazaar (bzr) and would like to use Ubuntu as a guinea pig for large scale deployments. At UDS Prague, James Westby gave an interview about using “distributed version control systems” (DVCS) for coordinating development.
SVN is the runaway winner here, but for both Ubuntu and Debian's sake, I don't anticipate this to last. SVN doesn't do enough to help developers with the merge problem. Git is quickly on the move, while bzr is standing still when it comes to adoption. The elephant in the room in this graphic is packages with no version control whatsoever. I imagine that widespread DVCS in Ubuntu is expected to lead to more adoption within Debian. The graph also doesn't distinguish between packages who keep the whole source code in revision control versus those that only keep the debian/ dir.