Here is an article that discusses the criteria for determining whether identities of anonymous internet posters should be revealed. The Right to Anonymous Speech

For those too lazy to go to the link--the discussion concerns Doe v. 2TheMart.com, a suit file in Seattle Federal court--2TheMart.com subpoenaed InfoSpace to reveal the identities of those made anonymous posts to their forum, claiming that the posts caused their stock to lose value.

From the article:
First, the right to speak anonymously is protected by the First Amendment.  U.S. Supreme Court cases that predate the Internet have ruled that "anonymous pamphleteering is not a pernicious, fraudulent practice, but an honorable tradition of advocacy and dissent.  Anonymity is a shield from the tyranny of the majority."  Second, the right to anonymous speech must sometimes yield to other interests.  One celebrated Supreme Court decision holds that "The most stringent protection of free speech would not protect a man in falsely shouting fire in a theater and causing panic." 

To balance these competing interests, the 2TheMart.com court adopted a four prong test to be used in this and future cases: whether (1) the subpoena was issued in good faith, (2) the information in the subpoena relates to a core claim or defense, (3) the identity of the anonymous commentator is materially relevant to that claim or defense, and (4) information to establish or disprove the claim or defense is unavailable from any other source.