I was thinking about the unrelated article, Territorial Markings as a Predictor of Driver Aggression and Road Rage (summary: drivers who put bumper stickers on their cars are more likely to be angry and aggressive than those who don't). Cars are pretty anonymous, but by making a vehicle more personal, people get more defensive (and offensive!) about it.
What if the general assumption about the Internet is wrong? What if people are actually jerks on the Internet because they are usually not in an anonymous public space? Most people getting into flame wars are likely sitting at home, or at the office. If a stranger says something online that ticks you off, maybe you feel a little like this fargin' bastiche just barged into your living room out of the blue and yelled at you. No wonder you're upset: your sanctuary has been violated!
I wonder if people are less likely to be rude online if they are logging in at the public library where some of the usual social cues are present (even if not on screen)?