Typically about 20 degrees Fahrenheit (or -6 C) is when I look at the winds, humidity and if the sun will be out. Then I'll decide if I really want to ride or not.
It was 32 degrees this morning and wet, and I went out anyways. I did hit an ice patch running 27 MPH when I crossed a bridge when I was about 15 miles from home. I am not sure how I recovered without crashing. Something, something, rotational inertia? :)