Today I commuted to work for the first time since before Christmas, and the first time ever when it's been really cold. In the morning the first 20km in the city and the forest were fine, a light dusting of snow in places but nothing hazardous at all, it was a really nice ride. The last 10km were lethal though! Part of it was on roads, which were fine, but part of it was on cycle lanes which were untreated and like an ice rink in places.
I came off twice on black ice, luckily I was expecting it and only going slowly, nevertheless I couldn't see the ice and the bike was out from under me before I could work out was was happening. I bruised my palms a little and cracked a pedal but other than that I was fine. Fortunately there was nobody around to witness it!
On the way home I was very apprehensive, I rode very slowly on the cycle paths and avoided them as much as I could. It wasn't much fun for that portion of the ride, shame really as the rest was great.
I read some tips on riding on ice during the day, the main points seemed to be that you should avoid braking with the front wheel, let that do the steering. Try to keep your weight over the back wheel and avoid braking and turning at the same time. Test the ground every so often by lightly applying the back brake, you'll know if it's slippy and be able to react accordingly.