Hey Mike, thanks for the detailed response, and I agree that what you suggested would be a huge project, and possibly not worth it. I think you misunderstood what I was suggesting, though.
I'm not saying that we should scrap everything and redo it from scratch with Git. I think we should leave everything as it is, and then add an *option* to automatically sync individual WordPress.org SVN repos w/ an external Git repo (at Github, or anywhere else).
Right now many developers are using Git as their main VCS, and then just copying to SVN every time they make a release. All I'm saying is that we should automate that last step, instead of having to do it manually.
So, the process could look something like this:
1) Add an input field on the Developers tab of /extend/plugins/[plugin-name]. Plugin admins can enter the path to a Git repo (e.g., git://github.com/iandunn/WordPress-Plugin-Skeleton.git)
2) WordPress.org will periodically scan that URL, and use svn2git to sync any changes to the .org SVN repo.
3) So, anytime a new tag is pushed to the Git repo, it automatically shows up in the SVN repo with an hour or so.
That way, developers can choose to easily switch to Git without the hassle of manually copying each release to SVN, or setting up their own automated scripts.