Sometimes, when you use a smartphone, the best search result is not a web page, it's an application. That's probably the reason why Google added an OneBox for iPhone and Android apps. If you enter a query that includes keywords like "download", "application" or "app" on an iPhone or on an Android phone, you'll see a list of results from Apple's App Store or from the Android Market.
"You can tap these links to go directly to the app's Android Market or iPhone App Store page. You can also get a quick look at some of the app's basic details including the price, rating, and publisher. These results will appear when your search pertains to a mobile application and relevant, well-rated apps are found," explains Google.
Maybe Google will develop a full-fledged search engine for mobile apps, index reviews, show recommendations and allow developers to advertise their applications.