The Power Pack for iPod provides unlimited power whenever you need it, through an AC outlet. This innovation includes a 90cm, Belkin iPod Cable. The Power Pack allows charge from the same, powered USB port.
Its been confirmed to work on an Iphone 3G, so should also work on the 3GS as they use the same plugs
10% discount code too - MOREOFFPLEASE
Postage is £2.99 but add a few things, like one of these for 1p