What did that mean? Until, today's ruling, it essentially meant whatever federal prosecutors wanted it to mean in cases of alleged political or corporate corruption, providing them with extraordinary discretion (and leverage) in prosecuting or threatening to prosecute people who had little if any notice that their conduct might be considered criminal. As Justice Scalia wrote in a concurring opinion (joined by Justice Thomas and, in part, Justice Kennedy), "Many courts held that some je ne sais quoi beyond a mere breach of fiduciary duty was needed to establish honest services fraud."
Scalia would have reversed (instead of remanding Skilling's conviction), disagreeing with the majority's effort to save the honest services law by narrowing it to cases involving actual bribes or kickbacks. But the court's relatively liberal wing at least agreed that the honest services law was vague and wrongly applied to Skilling (Justice Ginsburg wrote for the majority) and its unanimity in "siding with Skilling," in part, should temper progressives' hostility toward what they might otherwise condemn as a "pro-corporate" or even "pro-corruption" decision (which will also benefit publishing mogul Conrad Black and possibly political vaudevillian Rod Blagojevich, among other alleged or apparent scoundrels).
But the Skilling decision does expose the result-orientation of the court's conservative wing. While opposing the majority's effective rewrite of the honest services fraud law and advocating an outright reversal of Skilling's conviction, Scalia states that he is not advocating invalidating the law on its face, (although he has characterized it as unconstitutionally vague) because Skilling has not asked for "facial invalidation" and because "I continue to doubt whether striking down a statute is ever an appropriate exercise of our Article III power."
Critics of the 5-4 decision in Citizen's United striking down part of the federal campaign finance law might wonder at this expression of judicial restraint. They might wonder too why Justices Roberts and Alito joined in the majority's decision in Skilling to narrow rather than strike down the honest services law, given their refusal simply to narrow rather than strike down the independent expenditures bans at issue in Citizen's United (which is all they were asked to do in that case anyway; like Jeffrey Skilling, Citizen's United did not ask for "facial invalidation." Moreover, Scalia acknowledges that "the universality of the infirmity Skilling identifies" in the honest services law "may mean that if he wins, anyone else prosecuted under the statute will win as well," suggesting that in advocating a reversal of the conviction but not a facial invalidation of the law, he is relying on a distinction without a difference to preserve an appearance of judicial restraint -- an appearance that is increasingly, transparently deceptive.