My latest Guardian column, "How to foil NSA sabotage: use a dead man's switch," conducts a thought-experiment for a "dead-man's switch" to undermine the system of secret surveillance orders used by American government agencies. If you're worried about getting a secret order to sabotage your users' security, you could send a dead-man's switch service a cryptographically secured regular message saying, "No secret orders yet." When the secret order comes, you stop sending the messages. The service publishes a master list of everyone who has missed a scheduled update, and the world uses that to infer the spread of secret orders.
This gave me an idea for a more general service: a dead man's switch to help fight back in the war on security. This service would allow you to register a URL by requesting a message from it, appending your own public key to it and posting it to that URL.
Once you're registered, you tell the dead man's switch how often you plan on notifying it that you have not received a secret order, expressed in hours. Thereafter, the service sits there, quietly sending a random number to you at your specified interval, which you sign and send back as a "No secret orders yet" message. If you miss an update, it publishes that fact to an RSS feed.
Such a service would lend itself to lots of interesting applications. Muck-raking journalists could subscribe to the raw feed, looking for the names of prominent services that had missed their nothing-to-see-here deadlines. Security-minded toolsmiths could provide programmes that looked through your browser history and compared it with the URLs registered with the service and alert you if any of the sites you visit ever show up in the list of possibly-compromised sites.