I have yet to find a cutoff point where I would not ride.

Dec 14th 2009, Edmonton Alberta - temperature was -46.1C / -51F and I was riding... this is the coldest weather I have ever ridden in and was pretty comfortable as my gear was right and my tyres were studded.

Have commuted 30 km a day in -40C temps although temps of -20C / -4C is more typical.

The outside temperature is rarely an issue if you are dressed correctly.