Apple has added a new feature called Wi-Fi Plus Cellular to its iOS 6 beta 4. The purpose of this feature is to give smooth online experience to user. So, if someone was using WiFi connectivity in the iPhone and somehow that Wifi signal became week or stopped responding then iPhone will automatically switch from Wifi to cellular data mode, so user will not face any downtime.
The Wi-Fi Plus Cellular allows apps to turn back to a 3G/cellular connection when Wi-Fi connection got disconnected. That means, this feature will keep all apps running smoothly. The feature is listed under Settings > General > Cellular in the latest iOS 6 beta. If this feature remains till end of the final release of the software then you will able to configure your iPhone, and your iPhone will automatically turns back to cellular data to keep your iCloud Documents, iTunes purchases, Passbook and Reading Lists up to date.
Source: Apple Insider