We've all wanted to strangle http://jammerfun.com/ cell phone jammer before. There's nothing worse than some jerk blabbing away on their cell phone while you're trying to enjoy a movie, meal at a restaurant, or a nice quiet nap on the bus/train ride to work. Actually, only hearing only one side of a stupid conversation makes us pay more attention than normal. Our brain expects information from both sides of a conversation, and it will try to fill in the blanks. You can try to send the offending mobile blabbermouth a few evil looks or the old "courtesy cough" but those passive aggressive gestures rarely work.
If you really want to shut down a dopey chatter box, then you'll need to acquire a cell phone jammer.