See Limited time offer.
American Express Membership Rewards in the U.S. is now allowing SPG Members to transfer from Amex MR to SPG at a 2:1 ratio (normally 3:1) from July 1 through August 31, 2004.
Eg 90k Amex points usually converts to 30k SPG. During this promotion period 90k Amex points would equal 45k SPG, or 50% more SPG points.
(Just cross-referencing, please converse over in the linked thread above so as not to segment the discussion.)