On my ride to work 2 days ago it was -5°c. Then on the way back home it was -11°c. I have rode my bike in temps as cold as -20°c. Cold is not the determining factor for me, snow is. Once the roads are covered in a thick enough layer for my tires to dig in I generally take the bus.

Now interestingly enough my experience is different from groovestew, I tend to be faster in the winter, given snow free roads. Must just be my determination to get home and thaw my hands and feet lol.