A few months ago I read an excellent article in New Yorker magazine on this very topic. It's a long but excellent read:
http://www.newyorker.com/printables/fact/060206fa_fact
Some of the posters in this thread such as Cold Steel and lk70 have already hit some of the important points from this article, but I'd like to add a few more.
It turns out, as a general rule, pit bull type dogs have excellent temperment. The American Temperment Test Society has tested 25,000 dogs for stability, shyness, agressiveness, and friendliness in the company of people, and 84% of pit bulls tested have passed, which puts them ahead of beagles, Airedales, bearded collies, and most varieties of dachsunds.
The mean dogs are the ones that have been cross-bred with larger, human-aggressive breeds like German sheperds, and then trained or reinforced to be aggressive towards humans by their trainers and owners.
And the "mean dogs" change over time - not because the breeds themselves have changed, but because different breeds become popular among people who want to own an aggressive dog. Back in the 1970s, it was German shephards, St. Bernards (rememer Cujo?), and Dobermans. Now it's pit bulls. What we're seeing now is not the "natural aggressiveness" of pit bulls - it's the result of people who want an aggressive dog choosing pit bulls, and then making them aggressive.
There are a lot of factors that go into a fatal dog attack. Often, the dogs are sick or hungry. The dogs frequently have a history of aggressive behavior. The victims are usually children who are physically more vulnerable and may have unintentionally provoked the animal. But the most important factor is that there are certain kinds of dog owners who want a vicious dog. In about a quarter of fatal dog bites, the owner was previously involved in dog fighting. The junkyard German shepard that would tear your throat out as soon as look at you, and the gentle, obedient German shepard seeing eye dog are the same breed. But they are not the same dog, because their owners have different intentions.
It is not right to condemn all pit-bulls because they appeal to a few owners who want to make them into aggressive dogs. I don't know the facts of this specific case, but the owner almost certainly bore a lot of responsibility for making his dog into one that would attack a human, and he should be punished for it.