If you've got Animal Crossing: Wild World already and can go online with the title, now might be the time to do so!

For what we're guessing is a limited time only, Nintendo are giving away a special Super Mario Bros. style coin to decorate your Animal Crossing house with! And attached to the present is a special message:

I hope you are enjoying your Nintendo DS. Please accept this New Year's gift with my gratitude.
From S. Iwata

Touching the coin will make the classic "ba-ding!" sound effect you get in Mario games when you pick up a coin. Apparently it's sellable to Tom Nook for 256 bells, though it won't appear in your catalogue for re-ordering, so we suggest if you're lucky enough to recieve this ultra-rare item, you hang on to it!