As of September 10, 2013, the Cards iOS app service is no longer available.
Cards ordered before 1 PM Pacific time on September 10, 2013, will be delivered, and push notifications will continue to work.
I've always seen Cards as something that Scott Forstall personally wanted (he announced it in October 2011) and that eventually got into the App Store. I guess that, in spite of iWork apps not getting a visual update for iOS 7's launch, Cards really had to go.
It was a cool service. I'm glad iPhoto for Mac still has it.