Along with the "yellow tint" screen issues and antenna problems, a few of the early iPhone 4s have slipped through Foxconn quality assurance and into the wild with reversed volume buttons. While they still operate normally (the top one increases the volume, bottom one decreases it), the physical buttons show incorrect symbols. Now, before you march down to your local Apple store and demand an exchange, I'd give it some thought. It's annoying to see the volume buttons reversed, but in a few years, this could be a collectible.
Remember those times where you think "man, if I knew X was going to be so expensive now, I would have kept it...?" This could be one of those times. Anyone having a similar issue? Give me a shout in the comments and send your pictures to tips[at]phonedog.com, and I'll include them in the article.