This thread is very theraputic. I'm a great Monday morning quarterback when it comes to these incidents, but often laugh pretty hard at what I actually say when the time comes. I strive to keep my mouth shut, smug in the knowledge that most of the people who mess with me would suffocate if they tried to. It doesn't always happen that way.
"Get off of my lawn" really sums it up for me. People are very territorial by nature, and yell at those outside their vehicle like they'd yell at the TV. I really don't think most people who do this would consider themselves a-holes, completely oblivious to the fact that those are real people out there, who could really die under their 200hp lazyboy.
Not only am I more patient when I'm on a bike, but I actually exchange greetings with peds/cyclists almost every day, adding to my good day. Its just not the same in a car.