NeoGAF and official Xbox forums. According to the Microsoft spokesperson, "this short-term promotion is offered to select Xbox fans in the US, Canada and the UK."
Going by the image, Microsoft's dealing out the promo to some of its "very best customers." As can often prove the case with offers like this, however, the criteria for who qualifies is seemingly random. For example, it's fair say the Joystiq staff's spent oodles of times with their 360s, and none of us received the offer. As Eurogamer notes, the original NeoGAF poster got the promo despite already owning an Xbox One tied to his Xbox 360 gamertag.