ok, I had this problem too (and i still have it now)
here's what you do to qualify for an exchange:
if you shake the phone, and you hear a rattling sound(like the camera housing), you can go down and have your iPhone exchanged. I had that problem so I got mine exchanged, no conditions attached.
However, my exchanged iPhone still has the rattling sound when I tap the camera lens(i searched google, found out its the autofocus lens), so yeah I think its normal.(you might not be able to get an exchange with this problem only, I had both the aforementioned and this)
-Lawrence