I don't think McKellan has signed for the Hobbit, but he has indicated he would return as Gandalf should Peter Jackson direct the film, and Jackson has said he wants to do and the two studios should get the deal done. I could see them eventually splitting the costs of production and of course the profits, but it could be some time before that happens.
BTW, Christopher Tolkien would never agree to let Sony whore out any aspect of his father's vision. The Tolkien estate is obviously worth millions, but could be worth a lot more if he didn't safe guard the property, which he does, generally, for the best. It was rumored that he was against the films at first, but of course he now thinks they hold his father's vision well.
Also, anyone seen Peter Jackson lately? The dude has lost A TON of weight. I saw him last week on the AFI Salute to George Lucas, and I wouldn't even have known who he was.