Cindy Sheehan's freedom of speech ends when she accuses the President of criminal conduct and killing her son. Such comments are not only bordering on libelous, but ARE libelous and slanderous.
In Milkovich V, Lorain Journal (1990) the Supreme Court in an opinion written by Justice Rehnquist ruled, that the First Amendment does not automatically shield expressions of opinion from being found libelous LINK
Thanks for the link!
:)