Keep them off in class, a lot of professors find it annoying to have their classes disturbed by them. In one extreme case, one of my professors had a policy where if someones cellphone were to go off, he would demand that the person leave. If no one fessed up, he would stop lecturing and leave. Frankly it was unprofessional of that professor just to leave that way. I don't care much about having cell phones on, but it's courteous to leave it off.
