iOS 6 beta adds 'Wi-Fi plus Cellular' setting to ensure reliable network connection for apps
iOS 6, beta 4 features a setting for Wi-Fi Plus Cellular
Underneath the main switch is a sub menu that allows users to turn cellular connections on or off for a number of tasks, essentially giving you the option to use Wi-Fi only for certain data-intensive features like downloading music from iTunes, using Face Time (which is possible using 3G on iOS 6) or syncing documents from iCloud.
Being a beta release, this feature is just for developers at the moment and could be altered or disappear completely by the time iOS 6 is officially launched.
source: MacRumors
Things that are NOT allowed: