I agree with Salsa. It's the same rule I have for outdoor runs - 10F or below is too cold. But, she lives in MI and I'm in MN, so our standards of cold are probably a little different than everyone else. As a kid, when it hit 50F for the first time (usually late spring) I'd throw my swimsuit on.

I would say it depends on your access to appropriate weather equipment/clothing, wind, and ice/snow factors. I don't have the right tires for the current road conditions so I'm stuck indoors on the trainer or in SPIN class, but I do go outside for runs as long as it's above 10F. You just have to dress appropriately. Believe me, once you get going - you warm up pretty darn fast and what seemed cold at first won't bother you a bit 10 minutes later.