The Apple.com page for the app has also been removed, now redirecting to iPhoto '11 for the Mac.
Cards, which initially debuted alongside the iPhone 4S, was designed to be an iOS version of the greeting card creation service found within iPhoto ’11. The app allowed users to create a card, which was then printed, mailed, and sent directly to the recipient by Apple. Cards sent within the U.S. were priced at $2.99, while cards sent elsewhere were priced at $4.99. postage included.
While it is unclear whether or not the app will be revived with an iOS 7 update, the wording indicates that it will be quietly pulled from the App Store. Currently, while new cards cannot be created or sent, old cards can be viewed.
Cards, which is free, is still available for download in the App Store for the time being. [Direct Link]
Update: An Apple support document confirms that the Cards app will be discontinued on September 10, 2013.