Media event set for September 9, launch likely ~10 days later.
Yelp iPhone Application Adds Augmented Reality Easter Egg
Download the new Yelp app (came out yesterday). So you shake your iPhone 3 times. That activates a feature called Monocle. A message should come up if you activated it. A blue box will come up saying "the Monocle has been activated." It will create a button in the top right corner. Now you should be able to look at the bars, restaurants, etc. Only works on iPhone 3GS.Augmented reality applications have been under development for some time, but Apple is waiting for the release of iPhone OS 3.1 to officially enable the feature, which explains why the augmented reality feature in the Yelp application was included as a hidden easter egg to avoid detection by Apple's reviewers.
Mashable has also provided screenshots and put together a brief video demonstration of how the feature works.