The master branch of the git repository is now up-to-date with the leopard branch, minus any leopard specific code (of which is there none at present). R-devel is required to get things to compile. The GUI code isn't available yet, I'd like to get things to a truly usable stage first. I haven't had as much time this week, other things have taken priority, but some minor stuff was added such as the beginnings of the preference panes (and the infrastructure to support them) as well as the start of the action menus. I also changed the graphics device plot selector to take advantage of available space. Perhaps I'll post a screencast of that soon.
In the meantime, people who aren't using OS X, but want to consider a similar GUI type of set up might want to look at the Omegahat CORBA stuff, which did something very similar a long time ago. The page hasn't even been updated since before OS X 10.0 was released. :-)