My downward limit is flexible, depending on how badly I need to ride, but generally I don't go out in anything cooler than 20. My main concern in the winter is the condition of the roads. I really prefer to ride the back roads, and they don't dry out as quickly as the main routes. IMO, riding around in salt/sand/slush just isn't worth it. Again, though, depends on how I feel.

Now there's enough snow on the ground to do some decent XC skiing, so I try to obsess on that for the month or two that it's practical to do so. It involves gear, so it'll do.