How cold is too cold?

[silver]

25-28 F with no wind is generally my lower limit for rides of 1-2 hours.

I'll ditto that.

[mimitabby]

we're all different.

If you're concerned about sheer physical limits, like will you die if you ride at temps below freezing, I just don't think it's a problem.
The stronger riders around here tend to not ride when it's freezing because of all the dampness that turns into black ice.

Me, below 40 degrees, I'm not very happy. having said that, I rode 70 miles this weekend at temps between 38 and 42.