Heel-bottom mounted cell phone charger

Wouldn’t it be nice to be able to charge your cell phone by just walking? Imagine a charging unit (primarily made out of piezoelectric ceramics/crystals) mounted at the heel-bottom of one of your shoes, pumps out electrical juice on every step you make. Guess this kind of chargers could be a lot more effective than hand cranking ones, because legs are stronger than arms. The cell phone could be carried in an ankle holster, so the wiring to the charging unit is shortened. Otherwise, the long wire between your cell phone and the shoe makes you look really weird. Of cause, you can always hide the wire in your pants. I think the same concept also works on the charging of mp3 player, PDA, and other mobile gizmos. How does this idea sound?