While of course one should always do as you are told by the cabin crew and never argue I think if you have an aviation GPS and want to use it during flight there is no real reason as such why you shouldn't.
So: you will do what the CC tell you to do and never argue ... unless you decide not to do what they tell you and to argue because you think that there is no reason to obey them?
Sorry I didn't phrase that very well- just to clear this up I didn't mean to imply that you could ever disregard CC if you thought it was safe to do so, I was just saying that personally I don't think GPS effects safety.
My interest in this thread is not because I want to use a GPS system during an airline flight but because if there is an interference problem that would have very serious implications for a lot of GA types who use handheld GPS alongside radio nav avionics.