If you thought Regen's Renu solar-powered iPod dock was unbelievable, get a load of its big brother, the Reverb. It's 1m tall and capable of cranking out 60 watts of power off of the sun's rays.
My guess is that you will need all of that space to generate enough electricity to play your music for 4.5 hours at full volume/12 hours at a normal volume. Of course, it takes 20 hours of outdoor sun exposure (or 40 hours indoor) to reach a full charge. If you are not patient enough to wait, the Reverb can be powered up via a regular electrical socket — although I would suggest spending your $US2229 elsewhere if you see yourself doing that frequently. Speaking of being patient, the Reverb won't be available until April of 2010. [Reverb via Gizmag via Inhabitat]