The local Target store carried the Energizer Energi To Go battery backup/charger, to which I just purchased. I wanted a product that was able to not only charge the iPhone's battery, but to be very portable. This product seemed to meet what I needed.
The unit comes with 2 lithium batteries. The units itself has a sync port for the iPhone to plug in. It flips in (for storage) and out (for charging). The sync port is adjustable, allowing different thickness of iPods to seat correctly. This part of it is somewhat troublesome...sometimes the connection between the phone and charger wasn't making good contact. Eventually, the iPhone would start charging. The unit itself is a bit flimsy, and can probably break if not handled properly (the whole unit is plastic).
When I plug in the charger, the iPhone recognizes it. A little blue LED starts to flash periodically. It seems that when the charging almost completes, the LED starts to flash a bit faster.
One thing to note when using the charger is that the iPhone can only fit in the charger. If you have a case for your phone (those cases made by Contour or Belkin, or possibly those silicone slip-ons), you have to remove the phone from the cases. The phone won't fit correctly.
From what I can tell from using this accessory, I can say that it seems to work for the iPhone.
The unit sells for about $30.