Timothy,
Cooking in sub-zero temperatures requires a jacket, or bourbon—preferably both.
As far as the WSM goes, I've not had a problem cooking in cold to very cold temperatures, although I have found that as the temperature drops, the length of time on the cooker increases. The one weather factor I've found to be the most problematic is wind, particularly in conjunction with cold temperatures, cold wind wicks heat away from the cooker. When cooking in these conditions, I use a jury-rigged windbreak.
In January, I smoked 80 pounds of pork shoulder in sub-zero degree weather—extremely unpleasant cooking conditions, but not a problem overall. I took a few pictures, found
-->here (scroll down to 1/15/09). At one point it was -10.4*F actual temp, without taking the wind chill into account. The extreme temperatures added a few hours to the cook, but the pork turned out just fine.
Enjoy,
Gary