Semi-official Rails GIT repositories, maintained by Michael Koziarski :
$ git clone git://github.com/NZKoz/koz-rails.git
Update 13-Mar-2008: Steve’s git-svn mirror of the Rails repo (see the comments)
git clone git://git.sanityinc.com/rails.git
Rails on GIT
Posted by stoyan
I’ve been publishing a popular git-svn mirror of the Rails repo for a few months now:
http://git.sanityinc.com/?p=rails.git;a=summary
Seems to be much more regularly synchronised with svn than Koz’s.
BTW, there’s a rumour that the Rails Core team is thinking about switching to Git…