I have 2 Raspberry Pi's (first generation) sitting outdoors as motion sensing surveillance camera's. The past winter, they frequenct below 0 degree Celsius temperatures. In fact, I took this screen shot of the temperature as displayed by the shell a few weeks ago when the temperature dipped to about -10 degrees Celsius outside.
The only problem I ran into was when it got too cold, the Raspberry Pi would just stop working. I found out that the SD card wasn't making connection with the Raspberry Pi. I guess the cold caused the metal connector to shrink back? I dunno. I wedged some paper in there and that solved the problem. I don't know if this is an issue with the B+ or Raspberry Pi 2.