I have to agree with everyone else. There is not much that you can do, except let them know that you're not going to be bullied by their "road rage".

I used to get really mad and lose my temper, but then I was defeating the purpose of riding my bike to work, which was enjoyment. I now just follow the rules of the road, and wave and smile when someone else loses thier composure. Besides when all is said and done, the cage-driver will be the one that looks like an idiot.

I've had a few times where I was "raged" at, and just smiled and waved back. At the next stoplight some other driver who saw the whole incident commended me for my actions. I felt like I had a small victory that day because I didn't give in to temptation.