"Ensure you receive at least ONE brithday card, by sending it youself! By the time your birthday comes round you'll have forgotten you sent it, and will be pleasantly surprised."
Also things like sending letters a year on, with a reminder of what you hoped to achieve that year etc.
I was thinking along the same lines. There are already services for sending emails into the future (futureme.org is one), but I think being able to do it with actual mail is a clever idea.
One commercial use might be for real estate agents sending letters to new homeowners 1/2 months into the future asking them how they're doing, etc. I think that would seem a bit more personal than just an email. I always enjoy it when I start receiving mail at a new residence!
(Of course, the time lag is removed to demonstrate the general principle without having to watch a several month long video clip. And it's Christmas, not Birthday, cards.)