We've known for a few weeks that an automaker would be holding an event with Microsoft at The Detroit Auto Show. We then learned, courtesy of Ford, that it would be them. Everyone believes a new Ford GT is coming to the Detroit Auto Show. Combine the two bits of news and we arrive at this plausible conclusion: the new Ford GT is the Forza 6 cover car.
We don't know for certain that this is what's happening because neither Ford nor XBOX/Microsoft will comment on what they're going to say, but a Ford invite clearly states they have "News from XBOX" followed, the next day, by a roundtable discussion on "The Art of Performance in Gaming."
It's also possible that the new Ford Focus RS or the Ford GT350 or GT350R will be on display and that those will be on the cover, but... we're going to keep believing in the Ford GT rumor until Mark Fields himself gets up on stage and says "all you people are fucking stupid."
So. There you have it. Fairly decent, well-founded speculation that this is going to happen. If it doesn't happen I'm happy to eat the shit for getting it wrong.