I own a Sandisk Sansa e280 mp3 player, which acts as USB storgage. It's pci id is listed in the file /usr/share/hal/fdi/information/10freedesktop/10-usb-music-players.fdi. Because of that, GNOME should detect it as MP3 Player and Rhythmbox should, also. But this is not the case. GNOME just shows it up as storge volume. When plugging my device into an Ubuntu machine, it is detected nicely by both, Rhythmbox and GNOME.
Works as of Fedora 9..