As I recall, there have been devices in the past that amplify the sound of portable devices using a system of channels as opposed to electronics, but the AirCurve from Griffin is designed specifically to work with the iPhone and the iPhone 3G. No power source is needed—the AirCurve directs sound from the iPhone speakers through a "coiled waveguide" that, according to Griffin, produces a sound akin to a set of desktop speakers.
To be honest, I'm pretty skeptical about that claim, but at least it won't break the bank at only $US20. Although, that is probably more than I would want to pay for a dock that may only to a decent job of amplifying sound and is not capable of charging your phone (unless you buy a separate dock cable). [Griffin via Reg Hardware]