According to the Consumerist, some buyers of the iPhone 3G from AT&T were offered a free in-car charger as a bonus.  However, these free chargers were quickly found to be incompatible with the iPhone 3G.

With the release of the iPhone 3G, Apple also changed the way the dock connector delivers power to the device.  The first generation iPhone and the previous generation of iPods retained the backwards compatibility to charge via FireWire; however, with the iPhone 3G and subsequent generation of iPods, charging over FireWire was removed.

Many third party accessories utilized the FireWire power leads to deliver power to the device; even first party accessories such as the iPod Hi-Fi also used this method to charge the players.  With Apple’s decision to “improve” the dock connector came also the side effect of rendering certain accessories useless for charging.

Manufacturer such as Scosche have developed pass through charging devices to retain backwards compatibility, of course this requires the purchase of yet another accessory.  As a word of caution, if retailers are offering you free accessories with the purchase of the iPhone or current generations of iPods, make sure you test the device prior to leaving the store before you're left with a useless piece of plastic.