Could the president sue me if I posted a message critical of him?
No. With any free-speech issue pertaining to the Internet, established rules of First Amendment jurisprudence apply. If you were to post on a Web site a message criticizing a renowned scholar, a movie actor, or even the president, you could not be sued for libel unless “actual malice” was shown. See New York Times Co. v. Sullivan, 376 U.S. 254 (1964).
If criticism turns into threat, however, the situation changes. Threats are not protected speech.