Via Scoble (http://friendfeed.com/scobleizer/e6e411b4/new-yelp-iphone-app-is-also-out-the... ), the latest version of the Yelp iPhone app adds an Easter egg: "Monocle" mode, which you activate by shaking three times. Live camera view, slaved to the compass, which overlays Yelp listings for the direction you're pointing.
 
I've tried it, and it works. Not many businesses around where I work, but it works. Can't wait to play with this some more, and I'll try to get a video demo up soon.