Better late than never. A snippet from the email has all the information you need:
To thank you, our elite members, we’re offering you 500 bonus Starpoints® per eligible night in September 2008. These bonus Starpoints are in addition to any bonuses for which you’re already registered.
Simply register by September 15, 2008, at http://SPG.com/thankyou500 and start collecting bonus Starpoints for all your September nights. Feel free to forward details to your friends and family so they can take advantage of this offer, too.
We know the airlines aren’t giving us any point perks so get ‘em while you can.