I believe Denver had a ban on them for a number of years but now allows them.
Miami tried it, but ran into so many problems identifying non-Pitbulls as Pitbulls that it was constantly being litigated on appeal. Finally, the Florida Supreme Court determined that a breed specific ban was, by its nature, too vague to be Constitutional. Dogs are a species with a wide range of variability in size, body type, etc. The concept of breed isn't actually scientific, so there's no way to determine it in a legal sense, apart from the showing of pure breed papers, which few of the accused were willing to do.
