In the past, I’ve written extensively about using Git with WordPress projects. I’ve focused primarily on Git as the primary development channel, with SVN (in this case, plugins.svn.wordpress.org) used for distribution only.
In contrast, I use Git for all my local development on the BuddyPress project. In this case, BP’s “official” history is in its SVN repo. My local Git repo is just a mirror. This setup means that you need a different kind of workflow, one that gives precedence to the Subversion repository. I’ll be using BuddyPress as an example below, but a similar workflow will work for any Subversion-based project where you want to do local development in Git.
(Side note: Mark Jaquith published a similar guide on how he uses Git to do WordPress core development. His process and mine are independently derived, but they are, of necessity, conceptually similar.)