I've seen quite a few, but being a road warrior, space/size and weight are paramount to me. And finding a 9 volt battery is pretty easy.
I originally made this for my old Palms. With a few resistors it works for the iPhone.
Simple design. A DB-9 hood with the ears cut off as the case, the ubiquitous 7805 as the regulator and a 9volt battery connector. (I used one from an old battery, its stiffer then the ones you would buy at Da Shack) Granted the 7805 is far from efficient, but it works and keeps the whole thing small and easy to build.
7805 5 volt positive regulator
100K SMT resistor (2 pieces) Size 2012
20K SMT resistor (1 piece. I didn't have a 20K but used a 10K) Size 2012
USB female connector (I gutted a device to get mine)
9 volt battery connector (I got mine from an old battery, worked best for me, plus its less in the landfill)
DB 9 hood plastic for easier modifications
Wire, solder, heat shrink, potting compound (optional)
Rat tail file
Dremel (though I used a milling machine, a bit overkill)
Soldering iron (hot air SMT with paste works best, but I didn't dig up that stuff for such a small project)
On with the show!
Gather your stuff. I didn't take pics of tools (should I?)
The USB connector:
Pre solder the pins for soldering the SMT resistors. I did it far from the end as not to unsolder it when soldering on the wires later. I also snapped off the PCB mounting lugs