Simple. On your Linux home computer enter the command:
at now + 24 hours 'echo "I should be back, send help" | mail your_friends_email_address'
When you return home, use the atq and atrm commands to kill the scheduled job before it runs. No need to invite third-party burglars to your home while you're away by using some unknown website to do what you can do for yourself.
There was a movie a while back where the guy claimed to be retired from the .gov and had all this dirt on people and had a computer system that would broadcast it to the media if he didn't reset the countdown timer every so often.
Seems like you could just as easily send the same to facebook/twitter/etc or e-mail than use a web service.