Sam I have carbon and aluminium. I choose to keep my carbon for the dry summer days, not because its more likely to wear in the winter but simply because its my choice.
My carbon bike cost me a lot of money and I may never spend that much again, so I like to make sure its pampered. But as the others say theres absolutely no probelms riding it in the winter.
And of course im too lazy to clean a bike after every ride! So I dont mind the old winter ride getting dirty.
Everytime I go into my garage I stare at my carbon steed and dream of summer days, not too long now.