As suggsted on the mailing list:

  1. Clean up SVN so that the trunk looks just how we want it, minus all the
    cruft, and plus a bunch of good organization.
  2. Copy the head over to Git as a new starting point.
  3. Make a last commit into SVN leaving a note about the new Git location.
  4. Turn SVN into read-only mode.
  5. SVN will stick around indefenitely as a source of historical info.