New York Times vs. Sullivan
The New York Times posted an add about how Martin Luther King Jr. had been arrested by the Birmingham police, and that people were crowdfunding money to pay for his bail. The commissioner of the Birmingham PD sued for libel, citing that the incorrect facts in the add, such as how many times King had been arrested, harmed his appearance.
Constitutional Issue = 1st Amendment
Court decided that the publishing of wrong information about public officials was still protected by the first amendment. This does not apply to when statements are made with actual malice, when the publisher knows these facts are false but puts them out there anyways.
The police commissioner lost the case, and the ruling forced some states to change their libel laws. This was because states such as Alabama did not require the prosecutor to prove the accused libel had caused them any damage, only that the info published had been untrue, which was now unconstitutional.
Today it is basically unheard of for a media or press outlet to be taken to court for a libel suit, because it is so hard to prove actual malice. Because of this media has a lot more room to comment on and criticize public officials, such as Fox News and Obama.
Our current definition of libel and actual malice, as described in this case, seem to be set in stone for the foreseeable future. The only thing that could shake this is if media began to abuse it's rights and publish crazy stories with very little proof in order to mislead the public.