Apple has internally seeded a prototype iPhone with next generation internals in an iPhone 4 design, according to 9to5Mac.
The actual next-generation iPhone is specifically said to not include the iPhone 4/4S design, but Apple is testing these new devices in older casings to throw off leaks. The purpose of the prototype iPhone that we heard about is to test a variation of the "A5X" chip in an iPhone. The A5X processor in the new iPad was specifically built to drive the new Retina Display, so that chip wouldn't make much sense in an iPhone.
The prototype iPhones are said to feature 1GB of RAM and be labeled as N96. They are for testing the new chip's performance, not release.