iOS 6 Reportedly Introducing New "WiFi plus Cellular" Feature
According to reports, Apple is introducing a new feature in iOS 6 called "WiFi plus Cellular". When enabled, your iPhone or iPad will automatically check the WiFi connection you are on and if it detects any loss of performance, it will then automatically switch to your cellular connection. This is useful especially if you are using free public WiFi and the connection starts to get bogged down.
However, it seems that this new feature is used mainly for syncing purposes for the time being as the list of "allowed apps" presently only includes: iCloud Documents, iTunes, Passbook Updates and Reading List.