Of the top ten most downloaded demos on Xbox Live Marketplace, nine of them were released before the game. It's a safe bet that the recently released Dead or Alive 4, which followed numerous months after the game's release, didn't fit into the model for success on Microsoft's Marketplace.
That model going forward will include demos being released ahead of the game and downloadable content (called 'add-ons' by Microsoft) within the appropriate window after release -- Microsoft recommends releasing add-ons between 60 and 90 days after a game's release. On the success of games like EA's Fight Night 3, Microsoft is encouraging developers to make sure that a demo for their game arrives before the game (right, Mark Rein?). With Fight Night 3, Microsoft saw a 40% conversion rate from users downloading the demo to users purchasing the game.