This topic has already been discussed to death. This box art is fake. Here is IGN's statement on it:
http://wii.ign.com/mail/
For those of you who have not noticed, retailers
always obtain fake box art for placeholders until something better comes along. Were none of you paying attention when Twilight Princess was coming out? For a long time, no one even knew the official title. Once the title was released, there were several versions of box art all over the Internet.
As for this box art of Brawl, I would be downright embarrassed for Nintendo if that were the final draft. It is a
horrible cover for the game. First off, the background is a bright blue flash. Nintendo does better than that. The characters featured on the front are only the characters whom were revealed thus far. Now we know about so many more that it would be silly to even think that this cover is final. As for the drawing of the characters themselves, they really are not that impressive. It is definitely the best art so far (and quite unique), but I know people who can draw that well (and use Photoshop to add excellent shading). It is
possible this artwork came from NOA, but I can guarantee that it is
not the final box art.