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. FaceTime can only be used via a cellular data connection on the iPhone 4S/5 and the third-generation or later iPad models. Also, while the main iCloud Photo Stream is still limited to Wi-Fi, Shared Photo Streams will work over a cellular data connection and cellular use can’t be disabled without turning off the Shared Photo Streams feature entirely.
Note that the default setting for most of these services is ON, so you may want to double-check that they’re off if you have a more limited data plan.