A couple things:
Some people are confusing dogs being aggressive towards dogs and dogs being aggressive toward people. Pits are terriers and so they do have a high prey drive - this can translate to not being good with small dogs that for all intents and purposes are prey. Because some are / were bred for fighting, they can have issues with dog aggression as well. However, even fighting pits are selected to be gentle with humans - it's no good for the handler if the dog turns on them mid-fight. I would strongly suggest looking at pbrc.net for more information - they of course are pit lovers but have a lot of statistics to look through as well. Before you start talking about "aggressive" breeds, look up some breed information and temperament testing results.
http://www.atts.org/index.html
I think some of the responses on this thread are a little intolerant - people talking about areas of town with aggressive dogs and bad owners. It's easy to stereotype people by how they look and where they live, but there are bad owners everywhere. Coming from someone who works in a "bad" neighborhood in Philadelphia, I find statements like this offensive. Having money or living in a "good" part of town does not make you a good owner. And you'd be surprised - many of the toughest-looking guys are looking for a Shih Tzu to adopt, and many of the little old ladies want a pit or a shepherd. It's not always a status symbol; most people just want a pet to love. If vets are banning specific breeds to get rid of a specific "poor, thug-like" type of owner, I think that's pretty sad.
And coming from someone who has worked with thousands of pits (since we're doing anecdotes), I find the vast majority to be friendly, loving, hyper, and a little obnoxious. Not aggressive. It is rare for me to be afraid of one of our pits - I use a lot more caution with shepherds, chows, and anything small. The scariest dog I've ever worked with was a yellow lab.
One more thing: part of the media hysteria about pits is the complete inability to tell one breed from another. Try this out:
http://www.pitbullsontheweb.com/petbull/findpit.html