I have yet to experience "too cold to ski", but I've never had the chance to ski when it's truly cold. The closest I've come was a few years ago at Searchmont when it was 35 below when I pulled into the parking lot. It warmed up a bit as the day went on, probably getting up to -25 or so. But there was bright sunshine and it was one of the best ski days I've ever had. No crowds!
As others have said, the snow slows down once you get to -20 or so. At -35 it's noticably slow, and I've heard that at -50 it's too slow to ski. Someday, I hope to test that hypothesis, in the meantime, it's never too cold.
Wind, OTOH, can make it unpleasant if there's too much. Yesterday it was 10 degrees with 40MPH winds and I almost went in for a leisurely breakfast after an hour to wait for better weather. Almost. I was seriously thinking about it.