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Although there was no explanatory note or press release included in the box (shipped directly from Moose Studios Pottery in Clovis, California), we assume this is to promote the $19.99 Limited Edition "Game of the Year" version of Plants vs. Zombies for PC. This new boxed retail version of the popular PC/iOS game includes a plastic zombie toy and an in-game tool, called the Zombatar, for creating your own zombies.
If you've somehow managed to avoid the PC and iPad versions thus far, the game is also coming to Xbox Live on September 8 and to the Nintendo DS in early 2011.
Other unusual video game promo items: