When I get behind someone who is weaving or in the left lane poking along at the same speed as the car next to them, it's usually a safe bet that they're on the cell phone. I think it's a lot more common than drunk driving and just as dangerous. It's sad that we need to pass a law to get people to stop engaging in behavior that is risky to themselves and others. They should know better.
A couple of years ago the PM talk show in Austin was discussing cell phones while driving. One gal called in and said there was nothing wrong with that behavior. Just as she was about finished, there were tires squalling and a loud CRASH. She hung up.