Review site Yelp apparently beat all other augmented reality App contenders in getting an AR enabled app onto the Apple iTunes Store. I was under the impression that we have to wait until the iPhone 3.1 OS release to get AR on the iPhone 3GS. Apparently this is not the first AR app on the App Store. French PresseLite managed to get one onto the French iTunes store called Metro Paris Subway a couple of days ago.

You actually have to shake the iPhone 3GS to enter the Yelp app's monocle mode to get the AR experience. Did Yelp sneak the AR feature by the Apple App Store reviewers?
In any case the Yelp iPhone App uses GPS, Compass and the Camera to display labels for restaurants, bars and other nearby businesses with Yelp reviews.
More details and a video of the Yelp AR iPhone App on ReadWriteWeb.