I have not found one either. In every case (no pun intended) the case, or hte back part in 2piece versions, needs to be removed.
Honestly, though, with clearance (or lack thereof, as there is only a 1 mm clearance around the body of the phone when in the dock) issues the only thing I can see working would be those clear shield / body covers that I have seen advertised. Furthermore, since they are in lots of little pieces b/c of the angular lines of the DROID, I don't want them as they still do not provide full protection around the phone.
+1 to anyone who finds a *good* medium or heavy duty case that can work in the MM dock - and double kudos if it also works in the Car dock (which has even less clearance).