(Image: Yerka Project)
On the Yerka bike, you'll never forget to take your bicycle lock with you because it will always ride along. It's built into the bike frame. The down tube splits in two and pivots away from the bike. The seatpost slides into holes drilled into the two pieces of the downtube, forming a lock around a solid object, such as a lamp post.
Juan José Monsalve, one of the designers, explains that the only way to steal the bike is to destroy the frame:
You can break a lock and leave the bike intact, with our system if you break the lock you are breaking the bike, making it useless to ride or steal afterwards.
A very determined thief might do so, but most criminals are more likely to abandon the effort.
-via Lawrence E. Forbes