Eugene wrote a little bit about Martha Stewart's legal predicament, and points out some errors in a Slate piece about the issue. What's of real interest to me, however, is that he writes near the end that:
Finally, it turns out that the core issue -- whether false statements of fact are unprotected when they're not fraudulent attempts to make money, defamation of particular people, or false statements to the government (e.g., perjury) -- is indeed not fully resolved. ...So, I'm not clear on the issue. Does the 1st Amendment protect "harmless" lies, or would a federal law prohibiting all lies (in a content-neutral manner) be constitutional? It would be unenforcable, sure, and bad policy, but even aside from those issues I would be astounded if such a law would be allowable.
Rather, the strongest First Amendment argument for some protection even of knowing lies is that in some contexts (a) courts shouldn't be trusted to decide what's true and what's false, and (b) the risk of error in such decisions might be enough to deter even true speech. See New York Times v. Sullivan, which suggested that even knowing lies about the goverment can't be punished at all; compare State v. Davis, 27 Ohio App.3d 65 (1985) (affirming conviction for knowingly making false statements in a political campaign) with State ex rel. Public Disclosure Comm'n v. 119 Vote No! Committee, 135 Wash. 2d 618 (1998) (striking down a law banning false statements said with actual malice in election campaigns).