"Dear Macy Gray, could you please get this to David Bowie." "Paul McCartney, if you run into Serena Williams." Surely the celebrities would make every effort. It's the mail. It's their duty.

Even more scary, that would be perfect for spam, because of course people would open it. If it were kind-of intelligently targeted, in other words with a plausible confusion of usernames (paul at gmail dot com, whoever he is)...