I've had people get nasty like that on the highway if I have more than 10 feet of space in front of me. Last week, this happened to me on the way home from work and the guy then tried to illegally pass me on the *right*. I hate to admit it, but I wanted to show the guy that Camry Hybrids aren't weak. I punched the gas as he got beside me and easily closed up the hole in front of me (These Camry Hybrids really have excellent passing power!). He ended up being stuck 4 cars behind me because of the slower traffic in his lane...
I get really sick of people trying to pass me like that when I'm going the SAME speed as the person in front of me (but leaving a car length or 2 of space for safety).