Previewed at WWDC, launching in the fall.
Possible Photos of Plastic Rear Shell from Apple's Lower-Cost iPhone [Updated]
Apple has been said to be planning to release the lower-cost iPhone later this year in an array of colors, with one report claiming that Apple would be using essentially the same colors as found in the company's line of iPhone 4/4S bumpers, although that report indicated that green appeared to have been excluded from the set. This green rear shell is indeed fairly close in color to the green iPhone bumper.
Update 10:03 PM: Nowhereelse.fr has spotted two more photos [Google translation] showing the same part also in yellow and red.