Just to say that this has happened. We kind of took the easy route and did a top skim of svn trunk and used this to seed our new repo. All existing branches for the 4.x and below will be maintained in svn. Narayana (i.e. trunk) will be maintained in git.
trunk svn is now locked read only in deference to: https://github.com/jbosstm/narayana, but the existing branches are working as they were, although of course it is possible to access the branches using git-svn if you prefer.
The website has been update with information on git: http://www.jboss.org/jbosstm/sourcecode - although to be fair, the most significant pointers to working with git are provided in the (linked on page) excellent "hacking on as7" article.
Note, I also created a separate repo for our quickstart/: https://github.com/jbosstm/quickstart. Again, this is as per AS7s approach.
Thanks to all for your feedback and suggestions on this! I will now raise this in the user forum where any issues with user documentation etc would probably be best placed.