Apple's yearly developer conference should see iOS 8, OS X 10.10, and likely some hardware.
Apple's Internal 'GameStore' Testing App Accidentally Appears in App Store [Update: Pulled]
GameStore product listings and in-app purchase confirmation for "Nitrous"
The app is priced at $0.99, with the three in-app purchases currently available through the app priced between $0.99 and $2.99. Additional in-app purchases shown in the single screenshot shown on the App Store page for GameStore show items as high as $26.99, but these other items are not currently visible in the app itself.
In-app purchases made through the app do go through and users' accounts are charged for them, but with no actual game supporting the content available, they have no functionality.
App Store page for GameStore app and Settings pane from within app
When the app's listing is viewed in the App Store from an iOS device, the posting date is listed as June 9, 2009, which was during Apple's annual Worldwide Developers Conference just ahead of the public launch of iPhone OS 3.0 and the debut of in-app purchasing. Consequently, GameStore may simply be an internal testing app for the feature that was somehow accidentally published to the public App Store over the weekend.
Update: GameStore has now been removed from the App Store.
Update 2: TUAW's Erica Sadun took a closer look at the app from a developer's perspective, concluding that the app was likely sample code for an in-app purchasing developer test that somehow mistakenly got approved by Apple.
What developers do is upload a working skeleton application to iTunes Connect. You do this with the full understanding that you'll be replacing or, for tutorials, rejecting your binary at some point in the future. Once uploaded, you can test your IAPs, and make sure all your purchasing processes work. Looks like the app was submitted in order to provide a live testbed and may have gotten approved inadvertently.
After consulting with the TUAW team, our take on this is "likely sample code accidentally deployed to App Store" by Apple and then quickly pulled once people took notice. TUAW reached out directly to the developer we suspect was behind the app upload before it got pulled.