My policy statement from my syllabus is below the fold. You'll note it's not really a policy — more of a statement of principle and an indication of the issue's seriousness. Feedback welcome!
A note on in-class technology use
Technology, particularly mobile technology, is a ubiquitous feature of our lives. I have no objection to your using laptops, tablets, e-readers, smartphones, etc., to perform class-related tasks during our class meetings. However, their use for other purposes is distracting to me and to other students. In addition, while I am teaching, helping you to learn is the most important thing I am doing. Consequently, I expect that during our class meetings, learning is the most important thing for you as well. If it is not, I would appreciate your leaving the classroom. I may contact you if I have evidence that you are using technology in class in ways that inhibit your learning or the learning of others.