Nameless in Cyberspace, Anonymity on the Internet by Jonathan D. Wallace, Cato Briefing Paper No. 54, December 8, 1999 (available from the Cato Institute--http://www.cato.org/) provides arguments in favor of permitting anonymity online, along with extensive citations from the literature on both sides of the argument.
For arguments in favor of censoring internet speech, see Cass Sunstein, "The First Amendment in Cyberspace," Yale Law Journal 104 (May 1995): 1757
For arguments against, and in favor of permitting
anonymity, see Lee Tien, "Who's Afraid of Anonymous Speech? McIntyre
and the Internet," Oregon Law Review 75 (Spring 1996): 117, 152.
in ACLU v Miller, 977 F. Supp. 1228 (N.D. GA) 1997, a Federal district court in Georgia invalidated a state law criminalizing anonymous and pseudonymous Internet communications.
In McIntyre v. Ohio Campaign Commission, 514 U.S. 334, 115 S. Ct. 1511 (1995), the U. S. Supreme Court invalidated an Ohio ordinance requiring the authors of campaign leaflets to identify themselves.
Buckley v Valeo, 424 U.S. 1 (1976), held that it was constitutional to require candidates to disclose the names of donors, in order to enforce campaign finance law--and since the case also treated campaign donations as a form of speech, one might conclude that prohibitions of anonymity would be permitted if for sufficiently good cause.
The Supreme Court of California, which in recently upheld a state statute forbidding anonymous mass political mailings by political candidates:Griset v. Fair Political Practices Comm'n, 884 P.2d 116, 126 (Cal. 1994) (upholding Cal. Gov't Code § 84305 (West 1994)), cert. denied, 115 S. Ct. 1794 (1995)).
A federal child pornography law which requires
everyone concerned with the production of pornography to ascertain
identity, age, etc. of all performers was upheld in American
Library Ass'n v. Reno, 33 F.3d 78 (D.C. Cir. 1994), reh'g en banc
denied, 47 F.3d 1215 (D.C. Cir. 1995), cert. denied, 115 S. Ct. 2610
(1995). Arguably, this provides an example of a restriction on
anonymity that was upheld because it was content neutral (but was
it?) and furthered an important state objective.