Content filtering for free wi-fi

Munk writes: My in-laws own a truckstop and want to start offering free wi-fi to their customers. Since the wi-fi would cover the restaurant and other public areas, they don't want folks surfing porn where other customers could see it. And as a side benefit, I would also like to be able to block P2P traffic that would use excessive bandwidth. Does anybody have an suggestions for a setup to handle this sort of thing? I would obviously like to use linux and other free software if possible.