The OpenNet Initiative has published a report on the diversity of filtering programs and their impact on international law, communications, and policy. The initiative is an ongoing research partnership by the Advanced Network Research Group of the University of Cambridge, the Citizen Lab at the University of Toronto, and the Berkman Center to monitor international Internet censorship

A starting point: legal implications of internet filtering (24.10.2004)
http://opennetinitiative.net/docs/Legal_Implications.pdf