It's disappointing to receive a new, beautiful piece of technology to have random rattles or buttons that have play. But the simple fact of the matter is that some units will have these due to allowable manufacturing tolerances.
Many people have a "wiggly" home button. Meaning, it has side-to-side movement when it's not pushed in. Or, when it is pushed in it has a "squishy" and "indirect" feeling to it. Comparisons to the 3GS have been done and it seems the wiggly home button is new for the IP4, but the rattly/loose sleep/wake and volume buttons are present on all versions of the iPhone.
When I was at the Apple Store, I noticed wiggly buttons on all of their display units, including every single one with a wobbly home button - the one that's touched the most by the user. To me, that type of manufacturing "tolerance" is more like a defect.
Of course, you'll probably get many replies from users saying their buttons are tight and don't exhibit any unintended movement. This is, as they say, the nature of the beast.