The toggle, if enabled, changes iOS’ default behaviour of always switching to a known Wi-Fi network from 3G, irrespective of Wi-Fi connection quality.
The feature would automatically detect the quality of Wi-Fi networks (speed, signal strength etc.), and choose to switch to 3G in case the quality’s not satisfactory. Apple’s one line description for the feature in the Settings app reads “Allow apps having trouble with Wi-Fi to use cellular data.”
The switching feature, it seems, isn’t applicable universally to all apps. Instead, Apple has a list of services for which the feature can be explicitly enabled or disabled. For everything else presumably, the Wi-Fi 3G switching behaviour would be the same as iOS 5.
The list of services for which the feature can be enabled includes iCloud Documents, iTunes, FaceTime, Passbook updates and Reading List. This means that all these services would stay in sync even if Wi-Fi is briefly flaky, seamlessly switching back to Wi-Fi when connectivity gets restored.
The option would ensure constant availability on FaceTime, which might be important for folks who use the service a lot. Sadly though, carriers might choose to charge a premium to enable FaceTime over cellular.
The “Wi-Fi Plus Cellular” toggle can be found in the Settings app by navigating to General -> Cellular.
With the new iPhone expected to have 4G LTE, this option makes even more sense, a indicated by the use of the term “Cellular” over “3G” in the name.