In light of Bill Patry’s description of a new DMCA case involving “circumvention” of a robots.txt file, including his door-knocking metaphor, allow me this bit of shameless self-promotion: Michael J. Madison, Rights of Access and the Shape of the Internet
44 B.C. L. Rev. 433 (2003), which talks about the problems of metaphors and “effective” technological measures, among other things.