And everyone loves a conspiracy theory.
They sure do. But as Sherlock Holmes (or rather Arthur Conan Doyle) said, "When you've eliminated the impossible, whatever remains, however improbable, must be the truth".
Is it impossible that the next iPhone made it to the wrong hands? No.
Eight weeks from launch, Apple has no choice but to have lots of iPhones out there being tested in the wild, for QC reasons, and they must have the same physical properties as the final product since they need to make sure that it holds together and works well in that particular shape and form.
Therefore the existence of a future iPhone in the wild is not questionable.
As for the guy from Apple losing the phone, well... not really
impossible, is it? Over the ages, drunken people have lost their wallets, keys, pants, weapons, cars, pets, spouses and kids -- losing a cellphone no matter how secret is highly probable. Given the statistics on people who accidentally drop their cellphones in the toilet when they take a leak, it wouldn't even have been improbable if Gizmodo had found the iPhone half flushed down the crapper, nor would it have been improbable that they had reached down there and picked it up when they saw what it was.
Improbable would have been if the Apple employee parachuted with the iPhone in his pocket, but the dog had chewed on his pocket so the phone fell out of his pant leg and smashed right through the sunroof on some Gizmodo dude's car, but still not impossible, just bloody unlikely.
IMO the story is quite pedestrian and not very mysterious or improbable at all.