How is this different from the tons of Anti-Bush merchandise, videos etc etc that's out there. How is this different than labeling McCain a warmonger?

And defamation laws wouldn't really do anything here:

Defamation of character legal definition of Defamation of character. Defamation of character synonyms by the Free Online Law Dictionary.

The probability that a plaintiff will recover damages in a defamation suit depends largely on whether the plaintiff is a public or private figure in the eyes of the law. The public figure law of defamation was first delineated in new york times v. sullivan, 376 U.S. 254, 84 S. Ct. 710, 11 L. Ed. 2d 686 (1964). In Sullivan, the plaintiff, a police official, claimed that false allegations about him appeared in the New York Times, and sued the newspaper for libel. The Supreme Court balanced the plaintiff's interest in preserving his reputation against the public's interest in freedom of expression in the area of political debate. It held that a public official alleging libel must prove actual malice in order to recover damages. The Court declared that the First Amendment protects open and robust debate on public issues even when such debate includes "vehement, caustic, unpleasantly sharp attacks on government and public officials." A public official or other plaintiff who has voluntarily assumed a position in the public eye must prove that defamatory statements were made with knowledge that they were false or with reckless disregard of whether they were false.
It's unlikely the court would rule in Obama's favor over this. It's a kid for one and this stuff is out there everywhere. There is a reason other politicians aren't suing a number of tshirt companies. It's not like Obama is the first one to get a t-shirt with slanderous remarks on it. :hippy: