Presslite, the folks behind Métro Paris [$0.99 - iTunes link] and London Bus [$0.99 - iTunes link] for iPhone let us know that their apps are now updated in the App Store to contain... augmented reality!
For those unfamiliar with the concept, basically it uses the iPhone 3GS compass, GPS, and live video (what the camera sees) to figure out where you are and what you're looking at, and then adds an information layer (text, icons, etc.) on top of the live video to expound upon what you're looking at. For example, bus routes and schedules could be layered over a street corner, hotel room rates and vacancy over a building, Twitter status in the direction of a person you follow, etc.
Yelp! 3.0 [Free - iTunes link] has apparently also gotten into the AR game but through an Easter Egg that is activated when you run the app and then shake the iPhone 3 times. Check out Mashable's video after the break!
(Thanks to Nicolas and Frank for the extra info!)