Two facts corroborate Gizmodo's story. One is that the iPhone was disguised to be an older model. If it were just a model designed to sit on a test bench, I doubt it would be camouflaged. If it were intended to be tested in bars, however, then that makes sense.
Secondly, the identity of the Apple employee was released. He has a great incentive to say, "someone came into my lab and threatened to stab me if I didn't hand over the prototype". But, he has not denied the lost-in-a-bar story, even though he has an incentive to do so.
I don't think Powell's public silence to date indicates anything other than the fact that Apple hasn't authorized him to discuss it in public. (Most large companies wouldn't under the circumstances, it's not an Apple thing.)
If the phone was in fact stolen from him, he has no reason to comment in public right now--his job is safe and his name'll be cleared by the investigation.
Secondly, the identity of the Apple employee was released. He has a great incentive to say, "someone came into my lab and threatened to stab me if I didn't hand over the prototype". But, he has not denied the lost-in-a-bar story, even though he has an incentive to do so.
So I think the "story" is probably right.