So the story on Endgadget is someone left their prototype iPhone at a NorCal bar and there are pictures from a prototype iPad at an Apple testbed which shows the same unit.
I'll let you decide but Engadget has the scoop (they rock):
Engadget's iPhone 4G?!?
iPhone 4G Proof?
UPDATE: Gizmodo did a more complete inspection and this looks like the real deal: