Another way to keep you head warm, but ventilated too: I buy large silk scarves at thrift shops and if it is just somewhat cold, I wear the scarf tied around my head/hair and put the helmet on top; the sild is thin enought to fit nicely under my helmet with the hair. If it is really cold I wear the on-the-head-scarf and I will wrap the helmet itself in a large scarf, thus blocking the vents. I find the second way very warm, but not too sweaty (at least for me!).

An added bonus: not only do people get to look at me and wonder what an "old fat lady" is doing on a bike, but they get to sometimes look at the scarf and have a good laugh!--and the bright colors make me feel good too.

If you are not into gaudy scarves, you could find some lightweigth silk in a solid color at a fabric store and then cut, fold and mutilate it to your own ways.