This would be nice, however, I know for myself, I'd probably never use such a thing. I don't see what the point would be in having a case that only fits the clipboard of the SB, since the clipboard can only really survive on its own battery without the base for a few hours. That'd be a lot of case on, case off, on, off over the course of a day. I also cannot imagine a case that would not interfere with the connection to the base unit.
I'm using a Booq Taipan Spacesuit sleeve, which I think is amazing. I do wish someone made a slim case for the clipboard, one that allowed it to be docked onto the keyboard without removal.