Apple has added a Micro USB charging adapter for iPhone to their online retail store in the United Kingdom. The adapter, which costs £8.00 (~$12.3o US), ships on October 14th and is compatible with the iPhone 4S, iPhone 4, iPhone 3GS, and iPhone 3G. It has yet to be added to the U.S. store.
Official Product Description: Use this adapter to connect iPhone to a micro USB cable to sync and charge, or to a micro USB charger to charge. The Apple iPhone Micro USB Adapter allows you to use third-party micro USB cables and chargers to sync or charge your iPhone. Simply connect your iPhone to the Micro USB Adapter, then connect a micro USB cable or charger to the Micro USB Adapter.