Ardica makes a vest that conceals a large, flat battery. It provides not only power to your gear, but heat to your body.
The makers claim it holds "11 cell phone charges, 20 iPod charges and and enough juice to run a dead device or power a GPS, PDA or any other personal electronic device" so long as it requires no more than 10 watts of power. Alternatively, you can have 9 hours of low heat or 3 hours of hot.
Product Page [Ardica]