I'm not sure if I i should file a bug (should I?) because gnome-unstable isn't ready yet, but at the same time thought this might be useful for some people so I'm posting here. Hope that's okay.
Rhythmbox is crashing when trying to play anything because of mixed gstreamer0.10 and 1.0 symbols. This is because rhythmbox still uses 0.10 but webkit is linked to 1.0. Building it with "--without-webkit" in the configure arguments, and disabling the context pane plugin works around the problem. (That's what fedora has done: http://koji.fedoraproject.org/koji/buil … ID=359488)
I was able to recompile rhythmbox with this flag and get things working again, so thanks for figuring this out!
I am pretty sure the gnome-unstable devs would like a bug report, especially since you have already found a solution.