It had not happened in a while because temperatures have been quite mild, but this week I experienced brake cable freeze for the first time during a cold morning ride. I documented my issue with my Avid BB5 rear brake in a post a year ago this month. It is time for me to once again use gravity to empty the water out of my cable housing.
If it's really cold out, and your pump is exposed to the weather, avoid a valve seal blowout by sticking the business end of your pump in your mouth and blowing into it until the rubber loosens up a bit. Tim calls this giving the pump a Lewinsky. Heh.