Not yet, this is being worked on (no time estimate.)
Rockbox makes itself known over USB simply so it can suck 500ma over the USB teat. If Rockbox did not it would only be able to pull 100ma - thus charging would be slower.
For all the players which run on a PortalPlayer processor (iPods, iRiver H10, Sansas) you still need to boot into the original firmware for disk access.