I don't know if there are any documented cases, but I do know of one case involving a Pomeranian. A 6-week-old infant was mauled and killed by a Pomeranian years ago.
CNN.com - Small dog kills 6-week-old girl in California - October 9, 2000
However, I should point out that it's rare for such a thing to happen. The thing is, all dogs are capable of biting people.
I would bet you that most of the pit bulls involved in these attacks are in fact bred outside of the circle. Do not buy pit bulls from backyard breeders, you are asking for trouble if you do. If one was to buy a pit bull, it is better to buy one from a breeder with a good reputation. Far too many pit bulls are bred solely for the wrong purposes. It's a sad fact of life.
Pit bulls, it's not a pure breed. It's actually a mutt. In fact, most people don't know what a pit bull looks like at all. The other day, somebody pointed out to me that the dog 20 yards away from us was a pit bull. I corrected her saying that it's a boxer. Just to show you how uneducated people are when it comes to dog breeds. A lot of people who reported dog bites claiming it was a pit bull end up not finding the dogs because they don't know how to identify a breed.
I know several people with pit bulls, none of them ever mauled anyone to death. They are awfully suckers though, they always crave for attention even though they get plenty.
Putting dogs on chains will make dogs more aggressive. Dogs do not like to be restrained to a chain in the backyard. It's different when you walk them on a leash. It's a real shame that there's so much misinformation regarding dog bites these days. It is often the owners who don't know how to take care of their pets, so they reap the consequences for their mistakes.
That's life.
The media has done quite a great job of scaring people causing them to act incredibly stupid.