This is a cute idea: earth dog tags
. "In case of alien abduction these dog tags may save your life. The crucial data an alien will need to get you back to Earth is die stamped into these dog tags."
Plus, they come with a money back guarantee: "should you ever be abducted by aliens while wearing Location Earth Dog Tags and not returned safely to Earth you will be entitled to a full refund of the purchase price."
But I think they should also sell non-Earth dog tags. You could give them to friends as a prank. Imagine the look on their faces when they get returned to Mars or Jupiter! (thanks to Stewart for the link)