If you're at all handy with a soldering iron, this is a fun project to
do what you need : Minty Boost! - USB charger for your gadgets
Tekkeon's TekCharge MP1550 is a beefier version of this; it will work with
two or four AA batteries, and provides USB. If you put rechargeables in,
it can also charge them via a secondary charging port. This has become my
solution of choice for backup power for power-hungry cell phones that
drain their batteries every other day. The flexibility of being able to
stop at any store and buy a pack of AA batteries in a crisis is great. For
me, it's passed the pocket-indestructability test of having survived more
than three months unprotected in my pocket, which typically does plastic
thingies in.
... JG