It isn’t as crazy as it sounds. Tobias wrote about this over on the German Spreadshirt blog (which is written in German). Scientists are working on a fabric that can turn physical motion in to a power source. This article from Science Daily details how it works:
“The fiber-based nanogenerator would be a simple and economical way to harvest energy from physical movement,” said Zhong Lin Wang, a Regents professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “If we can combine many of these fibers in double or triple layers in clothing, we could provide a flexible, foldable and wearable power source that, for example, would allow people to generate their own electrical current while walking.”
The article also points out that there is one aspect of this that scientists haven’t figured out yet - how would you wash a shirt like this.
According to Reuters, the fabric can only generate enough power to charge a small ipod or mobile phone.
http://news.yahoo.com/s/nm/20080213/od_nm/shirt_dc