Mischief is often an engine of innovation. Figuring out how to make things do what you want them to do -- not necessarily what the designers want you to do -- is both fun and illuminating. Gibson wrote, "the street finds its own uses for things," and sometimes that use is as a medium for pulling pranks (c.f., "Flash Mobs").
Bluejacking is the latest manifestation of this desire to make mundane tools of communication interesting. In brief, it uses the "bluetooth" short-range wireless protocol built into an increasing number of mobile devices (everything from cell phones to Powerbooks) to send a short message to an unsuspecting recipient. Most of these devices are set to allow the transfer of virtual business cards and the like; "bluejacking" piggy-backs on this, putting pithy comments ("Nice shirt!" "ur cute" etc.) in place of business names and numbers. There's no way for someone to actually take over a remote device in this way -- "bluejacking" sounds more ominous than it really is -- but reactions to having odd little messages popping up on one's cell phone range from amusement to confusion.
The practice is relatively new, as bluetooth has only recently become a commonplace feature on mobiles, and seems to be more common in Europe than in the States. This is not terribly surprising; mobile phones are everywhere in Europe. It also seems to be a habit of the young.