This is what Amazon has posted as the Halo 3 boxart on their Halo 3 Product Page. It has the Microsoft Games Studio Watermark everywhere, and it’s art we haven’t seen before, so I’m leaning toward this being official.
I like it. It seems much more action packed than the previous covers. I believe that this cover is available for the Standard Edition and the Legendary Edition. Unless the Limited edition changes its cover slip.
Hopefully we’ll see a higher-res, watermark free version in the near future.
EDIT: Looks a lot like this: