Trawling Apple 's patents  is a rewarding  and frustrating game, since you never know what'll make it into reality . Rarely do you earn an "aha" moment. But this time, we may have struck a vein of relevant info--a new patent could explain the new iPhone's ceramic back piece.
The new application is a revisiting of an earlier patent where Apple demonstrated how it may build photovoltaic solar cell technology into the front of the device, inside the display screen. Now Apple's demonstrating how the solar cell electronics could be concealed behind the rear layer of the complex sandwich of display tech, semiconductors and glass that make up a typical touch-sensitive product. The patent is detailed enough to demonstrate how energy collected by the cell could be used to drive the iDevice directly, or act as a trickle charge top-up to the device's battery.
But as the folks at Patently Apple  were shrewd enough to notice, the technology could also work on the back of an iPhone, if it were to sit behind the exterior face of the device so it was protected from damage. This setup could double the power-capturing surface area available to a solar cell system, but it would require a rear layer transparent to the right wavelengths of light. And then you remember that those leaked shots  and rumors of the upcoming iPhone for 2010 included mention of a strange glass/ceramic rear face ...
It's unlikely that Apple has included this tech in the iPhone we're expecting to see unveiled next Monday at WWDC. But since this iPhone version represents a serious redesign, we can expect it to hang around for at least one more iPhone revision. At which point a nice PR points-earner like solar power may make for a nice addition.