Never seen this message for a game before, but then again I never looked when Halo3 was released:
Amazon.co.uk Availability Announcement
Due to exceptional demand for this title, we have already taken orders to fulfill our release day allocation. As a result, Amazon.co.uk will not be taking any more orders on this product until we have more stock. We would like to assure customers that all current pre-orders will be fulfilled. Please check this web page regularly for further updates