I thought of this idea for a new bicycle lock. I call it a drivetrain lock (or drive lock for short). It designed to deter theft by eliminating the use of the bike as the getaway vechile.
Basically you take a very small padlock (like a luggage lock) and attach it to your bike's chain. On a single-speed it will flip the chain off the bike. On a geared bike it will lock up the rear derailleur.
Of course, I don't want to advocate people lock their bikes carelessly. There are only a few limited situations where it is useful:
1. For messenger use or errands. For all those times when you have to leave your bike within site but unlocked.
2. If you put your bike on the front rack of a city bus. Use a drive lock first, and it will stop the "grab and ride" thefts of opportunity.
3. It also makes a pretty good supplementary deterrent for short-term use. Like if you only have a cable lock and need to stop in a store for a few minutes.
There's some obvious disadvantages with this type of lock- people can still pick up the bike and walk off with it, it has the potential to damage your components (particularly on a geared bike), and it can be easily defeated with bolt cutters.
I still like the idea of a lock that prevents a bike from being ridden. In Japan they use the O-lock for this purpose. I'm sure there are other variations on the concept.