I would happily ride to work in the winter snow, but they dump huge amounts of salt on our roads. In fact, if there is even a chance of snow, they will pour some sort of liquid sodium solution on the roads, and then follow it up with rock salt. I just can't destroy a bike by riding in that stuff.
So, my limit is any temp above 32 fahrenheit, rain or shine, as long as the roads are not icy.