Xbox team published the following newsflash:
Summer of Arcade may be over, but the great deals on Marketplace just keep coming. During October, spend 2400 Microsoft Points on Arcade games and/or game add-ons and you’ll get 800 Microsoft Points added to your membership as a thank-you gift. Spend your bonus points on anything you want—game add-ons, avatar accessories, or even another Arcade game!
Purchase at least 2400 Microsoft Points worth of Xbox LIVE Arcade and/or Game add-on content between 12:01 A.M. Eastern Standard Time (EST) October 1 and 11:59 pm EST October 31, 2010 and receive 800 Microsoft Points added automatically to your membership. Limit one gift per person. The free points will be deposited automatically into your Xbox LIVE account after the end of the promotion period. Please allow four weeks for delivery, on or around November 30, 2010.