The WordPress core, along with a few dozen plugins and themes, already have official and unofficial mirrors on Github, but they're sporadic and not always maintained. I think it'd be great to have official Git copies of the plugin and theme SVN repositories, to go along with the core repo that's already in place.
I think building WP sites in a Git repo -- like Mark Jaquith's WordPress Skeleton project does -- is a great way to manage them, but you end up having some plugins/themes in a Git submodule, and others just as regular untracked files, because there isn't a Git copy of the SVN repo available.
It seems like Git is becoming the de-facto standard for VCS, so there'll probably be more and more desire for something like this over the next few years. I'm already seeing developers use Github as their main repo, and then just copying into the WordPress.org SVN repo when they're ready to release.
The copying from SVN to Git could be automated with something like svn2git. The Git repos could be on Github, or a new git.wordpress.org site could be created for them.