The announcement for the bundle promotion in the US came from an update on Xbox Wire on Thursday. Between 7th-13th September people who purchase an Xbox One can choose a game to take home completely free, from participating retailers.
This offer does come with some terms, of course. The free game offer is for purchases of Xbox One and Xbox One with Kinect, plus it's only applicable to one of two bundles - Xbox One Madden NFL 15 and Xbox One Forza Motorsport 5.
The free game you choose must be new, disc-based, and it's retail value cannot exceed $59.99.
If you were planning on making the move to an Xbox One soon, next week may probably be a good time as Destiny launches on Tuesday too.