I've no reason to think this is fake, there was talk of a TDU style Forza game to be added to the franchise which had been dubbed Forza World up until now. Microsoft obtained Bizarre (responsible for PGR series) and some of their staff, IIRC one in particular being the sound guy hence why Forza 4's sound is even more epic than 3's. T10 had also hinted at some big news in one of their recent pit pass pieces on the Forza forums.
If it has the same physics as the current Forza series but set in open world like TDU this could be completely epic. I guess E3 will be the place we might get some more info and possibly footage. Can't wait, especially given that there are a lot of rumours that we'll see the first announce of next gen hardware from Sony and MS in a bid to make people think twice about adopting the Wii U and instead wait for the "core" platforms to arrive.