Nintendo's newsletter and official website has stated the following:

One of six very unique Animal Crossing: Wild World virtual gifts are awaiting you at your nearest Toys "R" Us store July 23-30. Trade 'em, sell 'em or decorate your home with 'em. Your Friends will be Mario-red with envy.

Hmm..."Mario-red," with envy? Wonder what that means...could this be a new wallpaper?

Here are the details on recieving your gift:

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1. Insert your Animal Crossing: Wild World Game Card into your Nintendo DS. Power up.
2. Purchase Bottle Mail at Tom Nook's Shop.
3. Write a message and release your Bottle Mail into the river or at the beach. Save and end your game.
4. Visit the specially marked Animal Crossing: Wild Word promotion in the video-game section of your nearest Toys 'R Us July 23-30. At the main menu, tap Tag Mode.
5. Watch and wait for something to arrive in your town!
6. Restart your game and comb the beach for Bottle Mail containing a special rare item!

[source nintendo.com]