Okay, so the tape doesn't quietly self-implode in a puff of white smoke. That would be sooooo analog, after all.
The "Mission Impossible" gang would love this Web 2.0 application that basically does the same thing. You might find it handy too.
It's called Privnote. It's free and simple (my two favorite qualities in a web app).
Simply type in your note in a post-it looking box on the front page of the site. Click a box whether you want to be sent an e-mail that your note was read. Click again and you get an URL. Cut and paste the URL into an e-mail to the one you want to send the note too (drawback: this wouldn't work with multiple recepients). The single recepient opens the URL, reads the note and its gone forever as soon as they close the page. They can save the screen or cut and paste the note somewhere else, of course. But, it's not in anyone's e-mail server.
Probably not secure enough for real cloak and dagger stuff. But, it looks like a good way to send around a little office dish and make it at least a little tougher for IT to track your every move.
Also on a practical note, some executives give full access to their e-mail to their admins. Here's a good back channel way to have some privacy.