Lower 40's typically is about my lower limit. I've ridden in the 30's but it just isn't enjoyable; it becomes an endurance test by then, so why do it?
I would be happy to ride at lower temps, but I don't have any heated gear. Heck, I don't even have any winter gear - I wear the same jacket at jacket, pants, boots, and gloves at 40 as at 100, I just open everything up in the summer and add base-layers in the winter. I do have heated grips though, and they come on pretty much any time it gets down around 60 if it's not sunny out. Since they're there, might as well use 'em!
'06 Speed Triple
'96 Adventurer Cafe Racer
and a bunch of other stuff
If everything seems under control, you're just not going fast enough - Mario Andretti