Okay, we know that the amount of negative space in the seat shaft is slim, and that the cable's gonna be similarly slim, but the winner of the most recent One Hour Design Challenge to design a better bike lock is RBAid's "TheftProof Bike" seat lock system. We liked that this solution embraces and exploits the behavior that people are already engaged in, and the fact is that this concept never failed to put a smile on the face of anyone we showed it to. So charm gets you half-way RBAid, but it would've been nice to see some iteration on external coiling, or another approach that preserved the "they're already taking the bike seat off" insight, but stood up to robustness constraints. (Oh, and bonus points if the spelling of "combonation" was ironic.)
Some notables: Special mention should go out to tadatadatada's "belt system," thinksketch's "integrated bike lock and pump," sprawlers' "don't lock your bike; ride your lock," kallol mohanty's "lock it graphically," and Jesse Daniels' "blue ink 'sposion!"