Thanks to everyone who replied. My riding buddy talked with a professional mechanic today who said the same thing as the first reply: the grease or oil becomes gummy when it gets that cold. Fortunately, the weather has turned a bit warmer since yesterday and we may have turned the corner weather-wise here in Wash. DC. But I will definitely have to remember the trick to getting the shifters to work, come next winter.

Again, thanks to everyone who replied--nice to know my shifters aren't getting worn out!