Luc landed detached playback a little over a week ago in r8282 (and a bunch of checkins before that).
The interface for it is a little rough and might change a bit over the next few weeks. It's something a lot of people have mentioned they wanted--David Ascher gave me a stern talking to about it at OSCON 2008. It's going to be a kick-ass Miro 2.0 feature.
I need to spend some time to get it working fully on Linux and Windows, but most of the code is there now and it works pretty well on OSX. w00t!