I've since used this as one of the key examples of smart contracts, in particular smart contracts that help enforce security interests (collateral). Other kinds of smart contracts include digital rights management (for copyright licenses), financial cryptography (for payment systems and other kinds of financial contracts), price-sensitive controllers, and a wide variety of other possibilities. More recently, some real-world examples of the "auto-repo-auto" have appeared, and it is now becoming more common. Deborah Yao describes some more of these:
If a loan was taken out to buy that car, and the owner failed to make payments, the smart contract could automatically invoke a lien, which returns control of the car keys to the bank. This smart lien might be much cheaper and more effective than a repo man.
Note that the process is far more manual and that there is much more attention to the user interface than I was thinking about in 1994. It's becoming increasingly apparent that a good user interface is often key to good security (phishing is another big example of this). Contracts are ultimately relationships between people, so a smart contract protocol needs to go all the way from end to end -- from person to person. Furthermore, as long as the parties have access to a good legal system, making certain important steps manual is a good idea if it increases flexibility and safety without too substantially decreasing security or user friendliness.
Starter-interrupt devices are becoming a popular way for lenders to ensure they get paid, and consumers seem willing to accept them to get into nicer cars, use a smaller down payment and qualify for a lower interest rate, according to device manufacturers.
...The companies make a variation of the same device: The units are connected to the starter and emit a brief series of sounds or flashes of light, days before the payment deadline. If the customer then makes a timely payment, he or she can contact the dealer for a new code that will allow them to operate the vehicle. Some devices are remotely controlled by dealers.
My thanks to Ian Grigg for pointing me to Yao's article.