Professors want to ban laptops from their classes because they're too big a temptation for the student to "goof off" instead of paying attention.
You know what? If you're students are having trouble concentrating on what you are saying, then you're probably teaching poorly.
I go to school. I do it ENTIRELY online. I don't have a problem concentrating on the lessons or the information. Why? I get to do it at my own pace. If I have questions, I set up a time to discuss with the professor or I seek my classmates help. I'm not locked in a room for an hour-and-a-half listening to someone with tenure drone on and on. I'd probably goof off, too, if I were.
Schools and professors need to figure out a way to grab the student's attention and make the lessons more interesting, or go the way of online schools and allow students to learn at their own pace. Perhaps instead of less computers, add one for each student, letting them learn at those workstations instead of listening to a boring lecture.
Regardless, the genie is out of the bottle. Even if you ban laptops, how are you going to ban cell phones? My Droid is everybit of the computer a laptop is. I can do virtually anything on it that I can on a full-size. How can you ban that? You can try, but it won't matter.
It's best to figure out a way to work with and take advantage of new technology rather than trying to restrict it.