For me, it's summer clothes (shorts, jersey, sandals) down to 50°F, and then I start adding jackets and tights as the temperature descends. I use half-gloves all year, but in the low 40s or so, the Moose Mitts go on the bike and I'll break out the Gore-Tex boots. (Or, if I'm on my road bike, the Lake winter cycling boots.) Much below freezing, I'll add a second pair of tights.
http://icebike.com/Articles/strategy.htm