Bluejacking:

A kind of practical joke played out between Bluetooth-enabled devices, bluejacking takes advantage of a loophole in the technology's messaging options that allows a user to send unsolicited messages to other nearby Bluetooth owners. A bluejacker will most likely camp out in crowded areas like shopping malls, airports and subway systems to find victims -- places with a potentially high percentage of people with Bluetooth-enabled devices.
Bluetooth technology operates by using low-power radio waves, communicating on a frequency of 2.45 gigahertz. This special frequency is also known as the ISM band, an open, unlicensed band set aside for industrial, scientific and medical devices. When a number of Bluetooth devices are switched on in the same area, they all share the same ISM band and can locate and communicate with each other; much like a pair of walkie talkies tuned to the same frequency is able to link up.
Bluetooth technology users take advantage of this ability to network with other phones and can send text messages or electronic business cards to each other. To send information to another party, the user creates a personal contact name in his or her phone's address book -- the name can be anything from the sender's actual name to a clever nickname.
Bluejackers have devised a simple technique to surprise their victims: Instead of creating a legitimate name in the address book, the bluejacker's message takes the place of the name. The prank essentially erases the "from" part of the equation, allowing a user to send any sort of comment he wishes without indentifying himself.
Bluejacking is imprecise, however. Searching for other Bluetooth-enabled hardware might turn up a list of devices labeled with a series of numbers and letters. Unless the bluejacker's target has chosen to publicly identify his or her phone, or it's the only Bluetooth phone in the area, the bluejacker may have a hard time messaging his or her target on the first try.