chezjfrey, i have noticed this as well. maybe it's a portland thing? i think people are secretly ashamed of their wasteful ways, and jealous that we are getting around so much more efficiently, and having fun while doing so. then, when we are keeping pace with them, they can't validate why they are driving, so they try to speed up out of frustration.
although i do think people here are more conscientious towards bicycling than in most places.