The first lot of Sonic and Sega All-Star Racing DLC is up on the Xbox.com Marketplace and here is the description of it.
Unlock All Characters & Tracks
Race past the competition and speed your way to victory with a whole host of Sonic & SEGA All-Star characters and exhilarating tracks. Unlock all 24 tracks and 22 characters from the start with this exclusive download. Winning has never been so sweet!
So it's pretty much just a glorified cheat code which they expect you to pay 400MSP for.
Source - marketplace.xbox.com/en-US/gam...