Using Garmin XT Mobile, the phone's radio has to be on. It does not use data (the cell connection and data transfer icons in the top menu bar do not show any data being transferred, nor does it start a connection to MediaNet). But the device uses aGPS, which means tower triangulation-- it needs checks to see the nearest cell towers the phone is connected to in order to get a location fix. So you must have a SIM in the phone and the phone's radio must be turned on, but it does not use the data connection. It merely looks at the currently active set of cell towers used for voice.
-------------------- Lisa Gade Editor in Chief, MobileTechReview