I built this a few months ago, but I have just now gotten around to posting it.  This battery belt is my solution to battery life problems of modern technology.  If you forget to charge your smartphone the previous night, you would typically have to go the rest of the day either phone-less (simply unthinkable!), try and scavenge power throughout the day, or carry a bulk portable battery pack.  The only reliable solution is the battery pack, but who wants to carry the bulky solutions available on the current market?  I came up with the idea of sticking batteries on the inside of a belt, making them invisible to everyone else (both due to the belt and the fact that typically my shirt covers my belt), and making the weight virtually unnoticeable (even though the belt is hefty, you don't really notice evenly distributed weight upon your waist).  The thickest part of the belt is only 1cm thick.  USB ports are located directly above both front pockets, allowing phones and other devices like iPods to be secretly charged in your pocket.  Both devices can be charged at the same time.

This is a belt consists of 8 stripped down JAPOD cellphone batteries.  All the batteries are connected in parallel to have a total capacity of 10.8AH (not mAH!) at 3.7 volts.  This belt could probably recharge a phone 6 or 7 times (i've never fully run down the belt, so I don't know) off of a single charge.  The batteries are connected using a three conductor flexible ribbon from Sparkfun.  One conductor is ground, another is Vbat, and the last one is Vusb.  Each usb port has it's data pins bridged and is fed 2 volts via a voltage divider.  Everything is covered up with a piece of fabric (that is starting to show wear) which is held on by rubber cement.

The circuit board is hidden behind the belt buckle.  Everything is thoroughly epoxied/insulated, but there are no saftey mechanisms on this device, making it fairly dangerous.  A safety circuit should be incorporated locally at each cell.  I have worn this for several months and nothing has gone wrong, but there's always a possibility of something catching fire with that much power around your waist; a fire on something strapped to your waist would be very bad.

If you are wondering, the red connector is simply a direct connection to the battery terminals in case I wanted to power something like a tron-esque suit directly from the batteries, bypassing the boost circuit.

Link for the batteries I used:
http://www.dealextreme.com/p/japod-bl-5j-replacement-3-7v-1350mah-li-ion-battery-for-nokia-5800-32398
NOTE: Don't be fooled for cheap chinese cell phone batteries, most can barely carry a charge.  These Japod batteries are an exception and actually have genuine capacity markings, even after months of use.

Conductive Ribbon used:
http://www.sparkfun.com/products/10172

The charging and boost circuit board:
http://www.sparkfun.com/products/10300

Information on usb data line voltage and charging for apple products:
http://www.sparkfun.com/products/10300
NOTE:  Most android smartphones just need the data lines bridged (no voltage necessary) to charge at the correct rate.