cell phone blocking

Does anyone know how or if cell phone jammers work?

I found a page that sells them:

http://www.globalgadgetuk.com/cell-phone-jammers.htm

What's cool is they sell not only industrial strength ones, so movie theaters and churches and restuarants can block cell calls, but hand held ones that theoretically work out to 15 meters or so.

If they were a little cheaper (the hand held is $250), I'd like to buy one and try it out. I was once in a movie theater in Maryland where someone answered their cell phone FOUR times during the film. That's absolutely absurd. I'm not one of those people who is anti-cell phone, I love my cell and carry it everywhere, but I turn it off in places where it's inappropriate. I'd like to carry around a cloud of silence with me that I could activate at will.