The other day I was swerved at by some dude in a pickup. I don't think he was actually trying to hit me, just spook me a little (he'd been stuck in traffic and construction behind me while I commanded the whole of my lane, the bike lane being closed off), so I gave him the double-middle-finger salute and figured that was that. Until I saw his brake lights come on. He pulled into the bike lane ahead of me and stopped. I stopped as well and pulled onto the sidewalk. We waited there for a while, until I decided he just needed another opportunity to swerve at me, so I started up again. He gunned his motor, turned right inches in front of me and sped away, but not before shooting me one of those tough-guy Clint Eastwood looks we all know so well.
Maybe the biggest problem in car-vs-bike altercations is the disproportionate response to actions on wither side. I considered (and still do) the middle finger to be a perfectly appropriate response to a swerve, but the implicit threat in stopping and waiting (will he get out? does he have a weapon? should I reach for my lock yet?) coupled with yet another swerve seem out of all proportion. When he cut in front of me, he came so close that, were I less skilled (okay, even less skilled) than I am, I could easily have fallen, maybe even under his back tire. I don't think I'm out of line in interpreting this as a threat on my life. Finger =/= threat of violence.
In this case and millions of others, I would say that the car itself plays a big part. I'm guessing that this guy (I actually recognized him once he drove by; I haven't seen him for years, but we used to drink together very occasionally) doesn't go around making random threats at people he passes in the street when he's on foot. But while safely inside a giant suit of armor, herded into a line of other such creatures and subjected to traffic, construction, etc, frustration coupled with an enhanced sense of power conspire to lower the limits of what he feels he's justified doing.
Ah, hell, I'm preaching to the choir, and probably not saying anything new. To summarize: cars aren't the whole problem, but they're well above 75% of it, in my opinion, by virtue of exacerbating the frustration of otherwise average law-abiding Joe (and Jane) Sixpacks, as well as enabling them to lash out with minimal threat to themselves and maximum potential for harm.