A presentation by Yaniv Shaked and Avishai Wool of Tel Aviv University at MobiSys2005 showed how Bluetooth devices can be hacked, even with security enabled, in 0.6 seconds. A story at TechWeb gives some details and the full paper is also available.

It's a real shame that things like Bluetooth are aren't designed with better security in mind from the start. It's just getting some traction and the convenience is unbelievable.