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 $20. 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]
I actually came up with something very similar to this about a year ago. Bastards stole my idea... It IS a great idea, whether or not some of you can see the usefulness.
The idea came to me when playing golf, riding around in the golf cart we wanted some tunes, so I used my iPhone and just turned it up so we could hear it... but we could barely hear it, esp. when driving. When it was my turn to t-off, I put the phone in the cup holder (speaker pointed down), it fit well, and to my surprise amplified the music a whole lot, to the point I had to turn it down because it was too loud.
It basically redirects / concentrates the sound-waves, kind of how the Bose wave radio works. Only this was a thin plastic cup holder, with a small hole in the bottom. It worked great, I wanted to patent the idea or something, but never got around to it... damn it.
You can easily construct something like this for yourself at home, it's very handy for camping, or just on the go stuff.... you don't have to be constantly charging your phone you know...