That's strange. I agree with Joey on this one. It would be blasphemy to put a sticker on a new, clean Macbook, and Apple would surely not do it themselves. On some machines, a sticker like that would be over a seam that, if tampered with to get inside the computer for some reason, would void the warranty. Apple does not do this to their products, though.
From where did you get your Macbook? If it went through the hands of a third-party store or something similar, they may have added the sticker when it came to them and may be able to give you more information.