Times Online: "Scientists have unveiled a new kind of 'ink' that disappears from a page 24 hours after printing, allowing paper to be re-used. When a document is printed on the reusable paper, the text initially appears similar to normal printed text – only in a shade of dark purple, rather than black. Eight hours later, however, the image is a shadow of its former self and after a day – much like the McFly family photograph in Back to the Future – it is gone completely."
A list of services that let you send various flavors of self-destructing emails.
A direct marketer's opinion: "I think self-destructible e-mail could help marketers create a sense of urgency for time-sensitive promotions. It would be just an added feature at our disposal and might be beneficial for time-sensitive e-mail campaigns."