I live in NY, and it happens all the time. Right after I get a car wash to get the salt off, the water from the wash freezes and I can't get in my car the next morning. My biggest problem, though, is the locks. I don't have a keyless entry system, though one is soon to come with my new alarm, so I need to be able to operate them. I can stick my key in, but not turn it. After much work to unlock it, if it is bad enough, the central locking system wont unlock the driver door (I don't put the key in that lock because it is broken on that door or something)
And after that ordeal, the door seals are frozen to my car. I am going to try treating them with De-Icer. Lucky for me though, I can always open the passenger door without too much force. The driver's door is a different story.