According to the CDC, there have not been any bioterrorism attacks using botulism in the United States that they know of. They also cite around 200 different botulism cases per year in the US. All of this aside, it is still a very dangerous and real threat to the United States.
While it has not been used against the United States, there are several countries that have been found to have possessed it. The Soviet Union and Iraq both contained the botulinum toxin with the obvious purpose of bioterrorism warfare. Iraq, for instance, had enough of the toxin to kill the entire earth's population times three.
The United States, however, is not innocent. During the 1940's and the rush of World War II, the US developed the toxin as a bioweapon as well. At the time, ally troops were given a botulism vaccine as it was assumed that both Japan and Germany were prepared to use the bioweapon. In fact, there was evidence that Japan had used it on prisoners of war before the conflict.
Luckily for the United States, counterterrorism agencies and the CDC keep an extremely close surveillance on any case of botulism. Every case is investigated to ensure that it wasn't involved in any bioterrorism. However, to this day no known instance exists of the botulinum toxin used as a bioweapon.