I'm in this same boat... I uploaded a quick YouTube video.
http://youtu.be/5qctM18D4RI
Doubt very seriously that I'm gonna get a replacement any time soon, but calling Apple Store nearest me now to see. Grr.
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I got three phones on my account today, one rattles and two don't ... so either I've got two broken autofocus lenses, or there's something wrong with the ONE phone. LOL.
Wow, that is a legit thud.
For a second I thought all these people complaining that their 5 had something loose, were just mistaking the camera rattle for a loose component, but after seeing that video..