According to this story from ABC News, cell phone jammers are illegal in the United States. Here's the relevant quote:
The devices are illegal in the United States, punishable by an $11,000 fine per day and up to one year in prison. The ban is supported by the influential cell-phone industry.
This is almost as annoying as cell phones themselves. How are we supposed to protect ourselves from all this cell phone yammering if we can't jam the signals? This technology is legal elsewhere in the world; then again, other countries aren't quite as under the thumb of the corporate overlords are we are here in the land of the free.
Here's the story: ABC News: Jammers Offer Solution to Cell Phone Disturbances