I hate to be a bad-news-betsy, but if Amazon's Brawl boxart is accurate, Sonic is probably not in the game.
I think we can all agree that Sonic is a big enough character to make it onto the box, especially for marketing purposes. That said, either the boxart being used for preoders is temporary, sonic is not in the game, or sonic is a hidden character.
I highly doubt Sonic is a hidden character, as much as I doubt Sonic would be included without being on the cover, but god almighty let's hope.
They don't put unlockable characters on the box art though, nor is the box art official. If Sonic is in the game, my guess is he'd be unlockable.
And another thing about the Mario and Sonic at the Olympics thing. I think it increases his chances of being in Brawl in anything, for two reasons. More people are going to buy Brawl than MaSatO. That's a fact. So if Nintendo wanted to increase their sales of MaSatO, adding Sonic would be a pretty good idea. People play Brawl, they see Sonic, they see that he's in MaSatO, and they decide to buy that. Kind of like how I'm going to buy FE: Path of Radiance and the Wii FE title just because of Ike. They've done this same kind of thing before with Roy to boost sales of the FE game he was in. They could do the same with Sonic for MaSatO.