Google started to index Android and iOS apps in June, but the results were only displayed as part of an OneBox. Now you can select "Android apps" or "iPhone apps" from Google's sidebar and restrict the results to mobile apps from the Android Market or the Apple App Store. Obviously, this option is only available if you use an Android or an iOS device.
Google uses a different way to rank applications than the app stores, so this feature could help users find new applications. Unfortunately, Google's snippets aren't always useful because they include the first sentences from the descriptions, which are sometimes used to announce new features or promote other applications.