Bluesnarfing, according to Daily Wireless:

to connect to the device without alerting the owner of the target device of the request, and gain access to restricted portions of the stored data therein, including the entire phonebook (and any images or other data associated with the entries), calendar, realtime clock, business card, properties, change log, IMEI (International Mobile Equipment Identity, which uniquely identifies the phone to the mobile network, and is used in illegal phone 'cloning'). This is normally only possible if the device is in "discoverable" or "visible" mode, but there are tools available on the Internet that allow even this safety net to be bypassed. "