Initially, OpenTable was the only app that could be used to make reservations. Now UrbanSpoon offers them as well, and customers are taking advantage. They're testing the idea in LA and Seattle, with 50 and 100 restaurants participating, respectively. Once you've eaten you can post a review. These are sorted by date, so those paying attention can see if a given place is on a slide or an upswing.
Even without these apps, I'll use my iPhone to search "coffee" or "hamburgers" with the Maps app to find a place in an unfamiliar neighborhood. Taking it that extra step -- placing a reservation -- is, as Spock would say, logical.