I have a verizon iphone 4S 16gb and mine rattles if I tap it near the lens, yes.
Should I be concerned? I got it yesterday. I wouldn't have noticed unless I didn't come across this ad. I've got a gameboy silicon case and I hope its enough protection.
I'm coming from an OG droid that was a little tank. It worked even after repeated falls, using it to listen to music in a hot, steamy bathroom and it even worked two days after it was completely submerged in water (I did the rice trick though).
No, there is nothing to be concerned about.
The OG droid was/is a tank though. Coworker has one and he's literally run it over with a truck and it's fine. It looks like hell but it's functional.