The National Highway Traffic Safety Administration apparently had a good idea on how to take a scientific approach to gathering quantitative data on how real-world driver actions lead to accidents: they placed video cameras in 100 cars and kept the film rolling for more than a year. That year saw over 240 drivers behind the wheel, 82 crashes, and over 700 near misses. They've now evaluated the video, generated raw data from it, and crunched the numbers, and the results go beyond confirming the obvious. Not only are distracted drivers dangerous, they're much more dangerous than previously believed.
Doing anything that requires significant attention, such as reading, applying makeup, or dialing a phone all tripled the risk of an accident. Using the cell phone was less distracting than dialing it, but tended to go on for much longer, balancing out the effect. To some extent, with age comes wisdom, as drivers in their late teens were four times more likely to get into crashes due to inattention than those over 35. All of this is roughly what I would have imagined, but it's interesting to get concrete numbers on it. I'd imagine that the insurance companies are going through those numbers as well, and will be adjusting their rates accordingly.