We use old icon for iOS6 devices. For those who don't like flat design we keep old good 3D icon.
And there is also a confusing bug in the App Store. It shows old icon for iOS7 devices, and new one for iOS6. But when app is installed it displays correct icon.
I looked it up on my iPhone and for some reason the search results showed a different (old?) icon.
Tapping into the detail view shows the latest icon — a tremendous improvement there! :-)