My button was loose. Drove me nuts. It wiggled about a half millimeter back and forth with even the slightest touch. You could see it was loose, and you could hear it if you shook the phone a little.
I went to the Apple store to compare it to the display models, which obviously receive a lot more clicks in a day than mine. None of them were loose.
I exchanged it, but it wasn't easy. The Genius Bar said it was within spec, which in reality, it probably was. The repair guy said the same. I had to be persistant, moving up the chain of command til I found a manager that agreed it was annoying.
The replacement one they opened for me had a misaligned bezel...it was raised up over the glass one side, leaving a sharp exposed metal edge. They opened another one for me, but warned me that it would be the last exchange they'd honor. Fortunately the 3rd one was fine.
The moral is, they may exchange it for you, or they may balk. If they do, the new one may have some other issue, it may not. They may offer a 3rd one, they may not. They may tell you no more exchanges, they may not. Exchanges seem to be at the discretion of whoever's manning the ship at the time of your visit.