I've seen an IGN Article say something about 3rd Parties that have or still going to be crossed over with some of the fighters. But here is the thing they said for Sonic's possible way in.
Sonic the Hedgehog
First Appearance: Sonic the Hedgehog, Genesis, 1991
Crossed Over: Mario & Sonic at the Olympic Games, Wii, 2007
And, of course, we must end with the clear and obvious selection. The fans are clamoring for Sonic to become a Smash Brother, and have been ever since the release of the previous game, Melee, back in 2001. The rivalry between Sega's Sonic and Nintendo's Mario is an epic one, and no one living in the early '90s would have ever guessed that the two characters would one day work together. But Mario & Sonic at the Olympic Games will see the two going head-to-head in the same game for the very first time, and that one confirmed cross-over is more than enough to fuel months' worth of speculation about the possibility of another. We'd be shocked and amazed if Sonic didn't appear in Brawl – call this one a near-guarantee.
I may be just be hoping. I believe IGN only have the more "Up there" people putting down the articles or else there would be 1,000 posts from new members about the game. Even though it kinda says speculating that M&SatOG assures Sonic's appearence. There are many things that could mean he is in Brawl, and if he is not in Brawl.
Possible ways Sonic is in Brawl
1. Sakauri stated Sonic as the most requested 3rd Party.
2. Many big guns in Sega & Nintendo want him in.
3. Sega & Nintendo are good friends now.
4. Mass popularity. (Heard Pit got in because of outstanding popularity)
5. Mario & Sonic at the Olymic Games states 1st game with Mario & Sonic in. But it realeases before Brawl. Thus not counting Sonic out of Brawl yet.
6. Many spots are still open, and if Mr. Smash Bros. keeps his promise for another 3rd Party andfinds the 3rd party character in Brawl dart board. The bigger target will be Sonic.
7. Sonic games have more success on Nintendo consoles then Sony and Microsoft combined
8. Sega uses their mascot for almost everything
Possible ways Sonic isn't in Brawl
1. Sega haven't recalled anything (Although they could be keeping it secret with Nintendo)
2. Sakauri may have already chosen his other 3rd Party(s).
3. Sega afraid that Sonic in Brawl will just lower down sales of M&SatOG.