There is the basic bike rule that the lighter the bike, the heavier the lock. Toronto designer Andrew Leinonen breaks the rule; in his new design, the bike is the lock. It's really clever; the lock is an integral part of the frame, and without it, the bike is unrideable.
Andrew tells Herb at iBikeTO:
Putting my mind to the endemic problem of urban bike theft, I realized that in a big city like Toronto or NYC, any lock (no matter how bulky or heavy) can only serve as a deterrent for a determined thief with the right tools. This challenge was the inspiration for the StayLocked bike, a design that integrates the lock directly into the frame. The bike's seatstays have been replaced with a U-lock on a pivoting joint. This is a 'scorched earth' approach; any thief that breaks the lock breaks the bike as well, rendering it unrideable and without value.
When riding the bike, the U-lock shackle is securely clamped into the rear triangle, but when the rider wants to lock up the bike, they simply unlock the shackle, swivel it into place around the post (or whatever) they are locking to, and slide on the lock body, as with a standard U-lock. Using the lock is instantly familiar, and doesn't require that riders change their behaviour or carry anything with them besides a key. It even saves weight, since you don't need to bring a heavy lock or chain with you (and a rack or bag to carry it with).
Because the U-lock replaces part of the frame, there is much less additional weight to carry around, and it eliminates the possibility of forgetting your lock. Furthermore, the bike thief doen't get anything for their work, the bike will fold up under them. Just don't ever lose your key.