Even though they say rust never sleeps, when it is that cold there isn't much actual rust activity happening really...it's when you get it warm and moist (such as in, say, a heated garage, or for that matter a car wash) that rust is happier. Of course, a good car wash will get most of the salt off, which slows the rust down a lot.

I saw a guy drive a van out of a carwash today, temp was -15F at the time...thought he was nuts. He'll be lucky to get out of the van, much less in it next time.