The popular emergence of apt-git?
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. The interviewer is a bit confused about how the Ubuntu flavors interact, so I think an explanation of DVCS and Ubuntu development is in order.
When talking about open source software in general, it's important to keep in mind the concept of versions. Each patch applied to a project can be thought of as a new version of the software. Generally projects release a new version every few months containing a bundle patches. Here's an example based on Xournal:
Xournal turns out to be a pretty good scenario so I'll keep returning to it.