This subject comes up among bicyclists every now and again as a chronic complaint. Experience has taught most of us that people throw water, soft drinks, ice and coffee at us; yell obscenities; toss cigarette butts our way; get their jollies from seeing how close they can get, then letting their horns blare, etc..
Now in all fairness, some bicyclists are twits. They hog the road, they wander across lanes, they disobey traffic rules, fiddle with their iPods, ride two abreast chatting with each other, etc. Those people are the minority, however, and they get under my skin as much as anyone's. And sometimes you need to honk at them just to get them to realize that they're unnecessarily holding up traffic.
Even so, some motorists seem to hate all cyclists, and most road cyclists end up developing a legitimate sense of paranoia over these things. A horn from behind to a cyclist pretty much results in a blast of adrenaline that instantly kicks in the the-jerk-behind-me-has-me-in-his-sites-and-I'd-better-gear-up-for-an-accident reaction.
Maybe if you're a block or two behind them, it wouldn't be so bad, but a far better alternative is to simply approach them slowly, then pass them carefully, acknowledging that they have as much right to the road as the next vehicle.
This is sort of a weird coincidental discussion for me. It wasn't more than two weeks ago that one of my cycling friends was complaining about the "hated [motor]bikers" that roar up behind him, rev their engines, honk their horns then race past him without even moving into the adjacent lane. In that conversation I was doing my best to provide the average motorcyclist's perspective.
If anyone is interested, here's a poll and comments on a cycling Web site about this very issue of motorists honking at them from behind: http://http://www.ecovelo.info/2009/...iewPollResults