It has been a source of much frustration to me (and obviously many others) that many of the iPod accessories will not work with the iPhone. Not sure for all of the reasons for this but one of the more irritating is that a large proportion of iPod chargers will not charge the iPhone. The worst thing is that there is no indication whether that particular accessory will charge the iPhone (unless marked of course) until you purchase said accessory and plug it in. I recently bought several iPod car adaptors on sale for $20 each only to find that one charged the iPhone and the other did not.
In researching this I came across some interesting information on iPhone charging adaptors and more specifically using USB for supplying power to connected devices.
USB has four pins designated +, -, D+ and D-. + is 5 Volts regulated, - is 0V or ground, D+ and D- are the data pins, send and return if you will. The actual current that can be drawn from a USB port varies on wheter the port is on a powered device, a hub or a dedicated charger, and also what revision USB the port is. What this means for the iPod and iPhone is that the iPhone requires more current and therefore the charger for the iPhone is slightly different due to the higher current required.
This article and its links provide some good information on how to adapt your regular USB adaptor to make the iPhone know that it can use it for charging. In short all you need is to add two resistors to one of the D lines between the + and - lines to tell it to turn on charging. In other words apply about 2.5V to one of the data lines. Some of the circuits use four resistors and others 2 but the outcome seems similar.
Of course there are other resons why your avereage USB car adapter will not charge your iPhone but you really need some skills in electronics to sort it out if it does not work first up.
I will be trying this out when I get home to see if I can get my wife's phone charging in her car.