Perhaps in part to save users from receiving frighteningly large bills for cellular data usage, Apple has traditionally limited many of iOS’ higher-bandwidth features to Wi-Fi connections only. Examples of such restrictions include iCloud Photo Stream, the 50MB download limit for the iTunes and App Stores and until recently, FaceTime.
In iOS 6, Apple has started to loosen some of these restrictions, allowing users to choose to allow certain services to use a cellular connection. These include iCloud Documents, iTunes (for iTunes Match and automatic store downloads), FaceTime, Passbook and Safari Reading List, and the controls to toggle cellular data off or on for each can be found both in their respective sections in the Settings app (e.g.
iCloud, Safari, FaceTime) as well as in a single list under General, Cellular. Toggling the option OFF for a given service will limit it to transferring data only when a Wi-Fi connection is available.
