THE ANNOUNCEMENT by the Federal Bureau of Investigation of a string of arrests in cyberspace child-pornography cases is the first real clue -- after gusts of prediction -- to the shape of future relationships between the new electronic media and law enforcement. One thing it suggests is that the FBI and other authorities already have a fair number of powerful tools for combating the dangers posed by cyberspace to children. One major advantage for law enforcement authorities, in fact, is related to the Internet's special nature as a medium of disembodied communication: its extraordinary ease of impersonation. FBI agents reported this week that they had made a number of arrests, including two locally, after posing as teenage girls and arranging rendezvous, using such youthful handles as "One4fun4u," who claimed to be 14 years old.