Why not? The majority of my students and student athletes use twitter REGULARLY, so why not use it as a tool to send out reminders, post assignments, and update followers on what is happening in your classroom or with your team. I know that several of my colleagues use twitter for their classes and have found it to be quite successful. I think that as long as the focus stays professional and does not become a place to just chat or hang out with students, then it could be very productive.