Here's how to get the game for free (which we've tested): You must be an Xbox Gold member. Head on over to the game's page and click purchase. Yes, it says that it's $20 (or whatever it is in your regional currency). Now comes the important part. Make sure you are paying with Microsoft Points, not with your credit card. You should see a final confirmation screen identical to the image above listing the game for free.
Enjoy! As much as anybody can enjoy Fable 3.
[Thanks to everyone who sent this in.]