Honestly, I think people are being way too hard on him.
How many people on this board would, if finding a next-gen iMac prototype, not immediately post it on MacRumors? Heck, people practically fantasize about doing this, making up mockups and fakes.
He made some effort to return it, and realized what he had and sold it to the journalist with the mindset of giving them an exclusive, not trying to move stolen goods.
He probably wasn't very familiar with laws on lost goods. He probably called Gizmodo and tried to get some money in exchange for a story, and was talked into celling it to them. Per the story:
A naive kid who stumbled into something bigger than him. 90% of the boards probably would've done something similar.
I probably would have tried to sell Gizmodo photos and videos myself before returning the phone. I'm not under NDA from Apple, I have no obligation to keep it secret after I return it.
I don't think it's worth ruining his life by arresting him for theft over this.