I've always thought self destructing email messages might be a good idea, that is emails that have a limited life and are permanently removed after being read or after a given period of time (read or not read).

10 Free Self Destructing Email Message Services

As we know, getting things deleted on the web isn't always possible. Google and other search engines do a good job of caching web content. Deleting your comments, bulletins or "wall messages" from a website doesn't mean a copy hasn't already been taken and stored somewhere else, somewhere you'll possibly never be able to delete it.