It seems rhythmbox is somehow set up as the default for handling folders. If i for instance type /tmp in the run dialog i get rhythmbox instead of a nautilus window. This can also be seen in the nautilus browser window. The information sidebar for every folder you visit has an "open in music player" button. Is this registration useful in some way? All i can see is it getting in the way.
Hmmm. Removing rhythmbox doesn't give me a nautilus when i type a pathname either. This is perhaps a deeper problem with the new mimetype system.
gnome-vfs2-2.8.1-6 has a fix for the general problem. But with rhythmbox installed we still get rhythmbox on typing in a folder in the run dialog.
Another nitpick: The Open with property dialog for folders contains just "Music Player" which is selected. This looks pretty strange. Especially when its not actually used when you open the folder.