In addition to a 22 percent year-over-year revenue decline, IAC announced during today’s first-quarter earnings call that it will acquire Urbanspoon for an undisclosed amount.

There are many possible synergies with IAC’s Citysearch — most notably mobile. Citysearch is pushing harder on its mobile strategy after seeing that its mobile users are 5X more likely to write reviews than its online users.

Urbanspoon’s biggest asset is clearly its iPhone app, which already pulls in Citysearch local listings and reviews. The app was one of our preseason favorites before the launch of the App Store, and is now one of the top 20 most downloaded applications of all time.

It’s also a great marketing story for Urbanspoon. The company has been around for a few years as an online local search and review tool. But it was the iPhone app that really put it on the map — much more so than any traditional marketing campaign could have done.

With its app, the company had the combination of an early mover advantage (way before there were 30,000 apps in the App Store) and a novel interface that did much more than port the online experience over to a smaller screen.

It’s a good example of the new thinking that’s required for local search tools (any tool for that matter) that have to be customized for not only a new form factor but also a new use case and type of user. Apple thought so too — and spotlighted the app in one of its commercials.

The Urbanspoon app is one success story in this respect (I’d put Shazam and Tap Tap Revenge in the same bucket), and today that value was realized for the self-funded company. Citysearch’s interest in the company was driven primarily by ownership of this app and its large user base.

The two companies will mostly exist side by side and cross-pollinate lots of content — much like Citysearch has done with Insider Pages. Urbanspoon meanwhile gets more capital and resources to develop its online and mobile offerings.

“So far life under IAC ownership looks a lot like life before the acquisition, except with better access to their resources,” Urbanspoon cofounder Ethan Lowry told us. Congrats to Ethan and team.