Kinetic cell phone chargers

Here's an idea that was long enough in coming -- a way to power your cell phone by dancing at your summer music festival of choice. It is not specified if the manufacturers would really prefer that you dance, or if you'll be allowed to charge your phone simply by waving your arms in the air like you just don't care, but either way I think it's pretty nifty. It apparently weighs the same as "a phone" and is the size of a deck of cards, and wraps around your arm:

Seems like putting it on your ankle and grabbing the energy from each step between the beer tent to the mosh pit to the bathroom and back again might be more efficient, but then people might think you had escaped your house arrest or something and you would no longer be cool. Or cooler, depending on the festival (Ozzfest loves the felon on the lam).