Found property as a general rule gives you the right of ownership as to all others but the true owner. If the true owner can be identified, it is still theirs.
There are exceptions, but I think that applies here.
The fact that Gawker (Gizmodo) paid $5000 for the prototype indicates that they believed it was real. And if it's real, it's obviously Apple's (Apple corporate) property. That means that Gawker is knowingly buying stolen property.
Gawker is trying to weasel out of this by claiming that they didn't know whether the phone was authentic when they bought it - which is total crap. Gawker isn't going to spend $5000 on a knock-off.
It's unlikely that much is going to come of this; the damage has been done and suing Gizmodo will just make a big media stink. Gawker probably knows this, and they decided to take the risk.