When the early morning temps hit 60? That's my wife's philosophy anyway. I've been trying to get her to commute with me, but she can't stand the cold mornings. She's going to have to get used to it since even in the Summer early morning temperatures here in the Puget Sound area can get a little nippy.
I see quite a few bicycle commuters here in Bellingham on even the nastiest days when the rain is pouring, winds are gusting 50MPH, and the temperatures are close to freezing. Those are the days when I often just catch a ride with my wife! Of course sometimes you just get caught in it, and that happens to me sometimes... but on the whole I find Winter here to be way more bicycle friendly than in most places. I'd rather deal with rain and temps in the mid 30s to 40s than deal with temps below freezing, ice, snow, salt on the roads, etc.
Now that Spring is in full swing (supposed to hit 67 this Saturday woohoo!), the bicycles are really coming out in force.
Sean