No matter how tough you make a bike lock, it seems like a determined thief can find a way to defeat it. This lock however is made from an essential part of the bike frame, so if you cut through it, you've ruined the bike.
Created by designer Andrew Leinonen after his girlfriend's bike was stolen, the StayLocked lock uses the top few inches of the seat stays as the lock, attaching them with a universal joint that lets you swivel them around a post. If a thief cuts through them, the bike will collapse under the rider's weight as soon as he tries to get on.
The one fatal flaw with Andrew's design is that it assumes the thief is smart enough to recognize what cutting the lock will do, that's probably quite a stretch.
While the StayLocked is a prototype for now, Leinonen is interested in selling his design to bike manufacturers.