Patches are currently flowing both ways as people commit to both repositories.
Git has excellent merging tools, including a way to step through the history of both the Subversion and Git sides, applying patches automatically when possible and asking the user for manual assistance when there's an ambiguity. Even if the repositories get horribly out-of-sync, which I'll try to make sure they won't, it'll only take a moderate amount of effort to get them back to a stable state.
I put a note in the subversion repository to this effect --- unless there are any complaints, I'm going to take the code out of the SVN repository and leave only a note. There's no point in spending any time merging back and forth.
For now, anyone who is interested in Arc is smart enough to figure out git.