"In beta 4, a new option can be found deep in the Settings application, under the Cellular tab, called Wi-Fi Plus Cellular. When enabled, it allows apps that are having trouble with Wi-Fi to automatically switch over to cellular data
With this feature, users on FaceTime calls will no longer be interrupted thanks to a poor Wi-Fi connection. And folks who are worried about exceeding their wireless data limit can obviously toggle the feature off and on as they see fit.
But its not just for FaceTime. It looks like Apple will be opening up this option to iCloud syncing, iTunes downloads, Passbook updates, and more. Itll really be interesting to see if they allow third-party apps to use the feature.
At any rate, the new Wi-Fi Plus Cellular toggle seems to be in line with Apples other enhancements in iOS 6. While it doesnt look like well be seeing any major new features this fall, it seems like well be seeing a whole lot of refinements."