My 3GS did this, my 4 did this, I know of 5 iPhone 4S's that did this and both of the 5's that I have in my possession do this.
I will note that I did have a iPhone 4 that the sleep/wake button was clearly rattling/lose that is not normal.
IMO it is a non-issue. I'd say there is a higher likelihood that your replacement will have a worse defect then a rattle that isn't even a defect.
What's wrong with slight play in the sleep/wake button? I mean, do you know what the long-term impact of this will be? I was gonna ignore it as it seems very firm and precise just makes a slight noise when in shake the phone. I think the vol buttons may have a tad bit of play too...