It's been just around 5-6 deg C on my last morning runs, which is low 40's. I've noted down the following: below 10 C/50 F, light/mid-weight wool jersey (Icebreaker 200 w/quarter zip recently) and full-length running tights. Below 5 C/40 F add (paper-thin) cycling wind jacket and thin wool hat. 0C/32 F add thin wool gloves (liners) and thin cycling vest. I carry a small hipbelt, though, since I semi-run-commute, so I always have somewehere to stash small items when I start to heat up.
I like wearing a jacket because it's very easy to pull off, usually I can pull it off and stash it while still running.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett