My previous mobile (Siemens) didn't seem to have any problems last winter. I remember carrying it in my pocket for the whole winter and the only problem I can recall is that the screen of the phone started operating more slowly than usual after being outside at minus temperatures. The mobile I've got now (SonyEricsson) hasn't been exposed to cold yet. The lowest temperature it's gone through so far is about -3C. Not very cold, I suppose.

Sweden will probably turn out way less susceptible to extreme temperatures than Germany...