Question: Fair use in Film Screenings

I enjoyed the Boston Eyes on the Prize showing in February ("Eyes on the Screen"), but have recently heard that open public viewings of copyrighted films may be actionable, even when admission isn't charged. Many colleges have a policy of not allowing community-wide advertising for showings of video on campus outside a classroom setting. Would someone please point me to the straight dope on when and how a video may be legally viewed and advertised among sympathizers and colleagues? (In the wider community, as was done with the Eyes on the Prize exhibition, not on a college campus.)