Cannon Mania ("devoted to the hobby of firing small cannon") has an excellent photo gallery and brief history of human cannonballs. From the Web page:
The first recorded story of a person actually being shot from a cannon was in England, circa 1877. And yes, it was a girl, (14) called "Zazel" (Rossa Matilda Richter). She went on to tour with the P.T. Barnum Circus. Unfortunately (as some stories are told), during one of her performances, she broke her back and had to spend the rest of her life in a back brace. Since then, dozens of brave (?) souls have been propelled from cannons at speeds up to 90 mph and as far as 201* feet at a height of 100 feet. They endure "G" forces of around 12 G's when hitting the net. More than half of these entertainers were killed while performing their act, most by missing the net. New York State even issued a law prohibiting anyone from being shot from a cannon.
Link (via We Make Money Not Art and TackyTimes Magazine, where there are more links)