Here's a creepy site that is based on a sound idea: JustInCaseIDie. On this service you can set up a message to be sent to a single recipient if you don't check in by a certain time or reply to a reminder e-mail. The idea is that, if you die, you can't check in, and your canned message will be sent. Said message could be a posthumous love note, a terrible expose, the combination to the safe in the house, and so on.
It's implemented in a pretty lightweight fashion, so I can't imagine it being used for anything serious. Potential users need to know that it works on London time only. That may be important if you're planning a check-in from a different time zone. And what if there's an e-mail hiccup? What if you're not dead, but just on vacation and not checking e-mail (and forgot you have a JustInCaseIDie trigger set)? You could easily have "Hello, I'm dead," e-mails going out that you didn't intend.
But the idea of a Web-based dead-man's switch is sound, and I'm surprised there aren't more travel or dating sites implementing a variation of this. When we're overdue to arrive at a destination (or home), it might be good to tell people where we thought we'd be. A delay could just be a late flight or a good date. Or it could be worse. But conditional alerts to our family or friends could prevent little scheduling hiccups from becoming bigger deals. All, hopefully, falling far short of actual death.