USB Fever has released the US$39.99 Power Pack with Click Stand for iPhone (http://www.usbfever.com/index_eproduct_view.php?products_id=2043).
With this item, you can “wear” this battery on your iPhone 4G, you can sync your iPhone 4G with your Mac or PC, even when the battery is on. With this external battery, you no longer have to hassle with the external battery with an external battery or synchronization, say the USB Fever.