Hmmm....I always considered a switch as someone who was aroused by both subbing and domming. If a sub doms another sub because her dom tells her to, and is not particularly aroused by the act of domming, but is aroused by the act of doing it out of submission to her dom's request, I'd say shes still a sub. If she does it out of the pleasure of domming, then I'd call her a switch. Now, that doesn't mean that any switch would sub or dom for anyone; I'm a switch but I would ONLY EVER sub for my DH. EVER. The thought of submitting to someone else (not counting fantasies) is not something I would ever be turned on by. I still classify myself as a switch though, because I get aroused submitting to him AND domming him.

Anyway, I have no idea if that rambling sentence makes sense! It is just such a grey issue- there is no mold that you fit into if you're one or the other- you just have to go with what your gut says, I guess!
-Phan