Don't be scared offThe really cold temperatures are the exception rather than the rule. I actually prefer skiing in January or February to March and April, if only due to the quality of the snow. Part of this might just be the fact that we're a little more acclimated to cold conditions (-10 F or lower), but the lack of humidity is also a factor. I've also never heard any talk by the package tourists (usually from Britain) complaining about the temperature when we've share lifts. And as mentioned before, we often have heat inversions or upper level chinook flows that keep the temperature several degrees higher on top of the hill than at Banff or down on the plains.
So don't cancel your trip, but do prepare in case we get a cold snap while you are here. I've found that on the coldest days I've skied (around -25 C), I've needed to have all of the following: base layer poly-pro tops and bottoms, polar fleece mid-layer tops and bottoms, lined ski-pants and ski-parka. If you add on a neckwarmer, fleece skullcap under your helmet, warm socks, and heavy duty gloves (think ski-mountaineering) you'll be toasty warm (or at least thoroughly protected). This is also when I make sure I wear ski-goggles rather than my usual sunglasses to cut the impact of lift-line windchill (frozen eyeballs). Most of the time the temperature is likely to be between 0 and 15 F, so I rarely get to pull out the full cold weather uniform and usually go with just the polypro bottoms and fleece top as a baselayer under lined skipants and jacket.
I can recall only about two days in the past five years when they've shut down Sunshine or Louise because of the cold. Those would be good days to spend up at the hot springs, or at several of the finer drinking establishments in the town. Just make sure you have a block heater on your rental car, and plug it in at the hotel (on really cold days you might want to take the bus to your hill, since there are no plug-ins in those parking lots).