cooldog, open up the bottom of your servo and have a look at the circuit board. Look at the labels on each of the chips, and google the datasheets. Each datasheet will list the temperature tolerances of each chip.
That will tell you how low/high in temperature the servo can go.
However if humidity gets into the chips, and then the water freezes, the chips could get damaged . . . check the datasheets . . .
I'd guess you will be fine at least to 0 degrees F . . . but I really don't know . . .