From the last time the subject came up:
Concealed Claws?
http://forums.sjgames.com/showthread.php?t=5480
The consensus seemed to be that if the claws are still visible when retracted, like a cat's claws, or any natural creature, that is part of the base advantage.
However, if they are able to be completely hidden, like nothing in nature, then you'd have to take the +10% Switchable enhancemen.