Given that tuition my school costs about $1/minute for every lecture attended, you wouldn't figure too many students would be interested in screwing off, but it happens. There's only so many hours of Tax Law you can take before your mind craves escape. But still, lectures for some subjects and some professors frankly are a waste of time. You can read the materials and commercial briefs on your own and get everything you need. Listening to rambling philosophical ruminations is a terrible waste of preciously limited supply of time. The ABA requires minimum attendance of 80% of classes, which is fine, but when you have the inevitable 12 hours of studying or outlining to catch up on, skipping classes would help.
One professor I had last year for Property Law banned laptops from her classroom. She was a rather feminist type. I heard that the reason she banned them was because a student the year before had a screen saver which featured a hot chick who would loose an article of clothing after every couple minutes. The guys in the back of the class would all be like "awwww!" when he moved the mouse to take notes, and the timer would be reset to fully clothed.
I have actually found it distracting when someone is playing WoW or whatever in front of me in lecture, but I don't think there should be a universal ban. Law classes are pretty intimate settings and usually everyone pretty much knows everyone. The vast majority of the time students in a class can just handle these problems one-on-one.