How cold is too cold?

Today is one of those days where I planned on running and I didn't.  So I'm feeling so extraordinarily guilty about, I'm turning to the Internet to justify my reasons.

It was 10 degrees outside today, but the wind chill made it feel like -5.  I had planned on running after classes, but I was shivering inside my wool coat, and my toes were thoroughly frozen.  So yes, I could have taken the miserable route and have gone running, but instead, I'm now curled up under my covers, looking up cardio kickboxing workouts.

I chickened out today.  But my question is, how cold is too cold?  At what point do you decide to let the weather get the better of you?