To whom it may concern: You are charged with the duty of saving humanity. Your mission is to fill out this guestbook successfully. You see, this is no ordinary guestbook; it contains a hidden bomb. Failing to fill out this guestbook could result in the bomb's release of billions of killer sea monkeys. And you wouldn't want that, would you? Good luck.
And for all you insecure superheros out there - no, your entry won't be posted on the Web for everyone to see. I get 'em myself. Now get to work!
If you're building giant commercial edifices with imaginary writer's blocks, here's a sample entry from a friend's guestbook.