I feel it's polite too. I feel quite rude if someone nods to me and I've been concentrating on something else and wasn't in a position to nod back, but obviously there's nothing else I can do in that situation.
One time I don't try to nod to people though is when they're going really fast round some twisties. It makes me wonder what people are thinking when they nod at me whilst I'm doing 90mph on some back roads and braking quite hard for a corner. It's pretty obvious I can't do it back! (maybe they're just being nice
)