According to the Oxford English Dictionary man bites dog (or man-bites-dog as the OED prefers) is an adjective of U.S. origin used to descibe a news story or event, "arousing interest because of its bizarre or unexpected nature".
The first recorded reference to the phrase comes from the Story of Sun by F. M. O'Brien published in 1918, which recounted how a former a city editor of the New York Sun named John Bogart once said to a young reporter; "When a dog bites a man, that is not news, because it happens so often. But if a man bites a dog, that is news." Thus the origin of the phrase is generally attributed to John Bogart, although it is quite possible that Bogart was simply repeating what was a commonplace saying at the time. In any case this has never stopped other newspapemen (such as the first Baron Beaverbrook) from claiming that they'd originated the phrase.
Man bites dog was also the title of the Belgian movie which won the International Critics' Prize at the 1992 Cannes Film Festival which according to Amazon "stunned audiences worldwide with its unflinching imagery and biting satire of media violence".
The phrase has also been utilised as the name for a number of musical groups, each of which no doubt believed they were being breathtakingly original. There is one band of that name from Penrith in Cumbria and another from the Coachella Valley in California (both of whom have profiles on MySpace), another jazz band that uses the same name, as well as a Moscow based post-punk band. There are probably more.
Man Bites Dog (PR With Teeth) is also the name of a Public Relations agency which describes itself as "a new independent PR agency with global expertise and a passion for delivering results that make a real difference to the bottom line". Based at Brighton in the United Kingdom its clients include Microsoft, IBM, and the Nottinghamshire Fire and Rescue Service, although quite why the last named should require a PR agency with "global expertise" is a question that the citizens of Nottinghamshire should no doubt be putting to their elected representatives.