It was 28 F when I left for work this Friday morning. The cold isn't really a problem but the frost could be. I felt my tires start to come loose from the pavement while going around a corner - an adrenalin boost on a two-wheeler...but a little too risky for my tastes. Riding a motorcycle on icy streets is probably a bit stupid, so I'll be trying to avoid that in the future.
Postscript - Saturday was nice - sunny and cold, but there was some frost in the shady areas so we spent the day going up North to cut down a Christmas tree (drove the pickup...not a bike). Sunday afternoon it was 36 F and sunny. I rode my Kaw about 70 miles - mostly on the freeway. It wasn't bad at all with the windshield and riding suit. I just need something a little better for my hands - maybe some Hippo Hands or something like that.
I tried to fire up the Sportster when I got home and that 20-50 oil was like molasses in January. It's suppose to be good down to 40 F, so it was a little cold for that weight oil and the engine just couldn't get spinning fast enough to fire-up, plus it's probably not the best thing for the engine bearings, cylinders, push-rods to run that weight oil when it's below 40. I have the trickle charger/maintainer on it now and hope we see some 40 plus weather here in the not too distant future.