Heh. Damn, that sucks. I found it completely by accident when I was perusing Kotaku. All sorts of people were saying, "this is legit, why else would it be 7GB" and others were saying "it's only the first episode, but like other game trials it downloads the full game so you can have it all at once when you buy it."
I've played a bit last night and then a good bit this morning and it's completely legit. MS twitter said it's only the first episode, so someone borked big unless it was a super limited-time (shady, but not so shady) deal to generate hype for Fable 3.... which I wouldn't be against believing.
Though in the end, I hope it wasn't so big of a mistake that someone gets a chair thrown at them. :-(