According to Livescience,
A five-year review of dog-bite injuries from the Children's Hospital of Philadelphia, published in 2009 in the journal Plastic and Reconstructive Surgery, found that almost 51 percent of the attacks were from pit bulls, almost 9 percent were from Rottweilers and 6 percent were from mixes of those two breeds.
Fans of pit bulls are quick to assert that a dog's propensity for attack depends in large part on its owner and how it is raised, and there's considerable evidence that owners of pit bulls and other high-risk dogs are themselves high-risk people.
A 2006 study from the Journal of Interpersonal Violence revealed that owners of vicious dogs were significantly more likely to have criminal convictions for aggressive crimes, drugs, alcohol, domestic violence, crimes involving children and firearms.
Personally I do find pit bulls to be a breed that seems to attract trashy people, and lots of pit bull owners do unfairly characterize their dogs as teddy bears who would never hurt a fly (in a way I've never seen an owner of any other breed do). Aside from the one source I posted as an example, I've seen loads more that consistently rank Pitbulls (and, distantly, rottweilers and German shepherds) as the breed guilty of the most dog attacks in the US.
What do you guys think?