[edit: I was just about to start a countdown for how long it would take for this promotion to show up on some of the less scrupulous blogs, when I saw that seconds before I published, over at Milepoint gconnery posted the same thing. Well played, gconnery, well played]
American Express has been quietly running a promotion for the past few weeks that I only noticed today. If you register your American Express Hilton HHonors or Surpass credit card, you'll earn an additional 6 HHonors points per dollar spent on your "room rate and charges to your room" at Hilton properties within the United States (excluding Puerto Rico and other US territories, possessions, and minor outlying islands) from November 20, 2013 to February 28, 2014, up to a maximum of 15,000 bonus HHonors points.
Interestingly, there is nothing in the Terms and Conditions excluding stays you've already completed, as long as you paid for those stays with your registered American Express co-branded credit card.
As a reminder, the no-annual-fee Hilton HHonors American Express typically earns 7 HHonors points per dollar spent at Hilton properties, and the $75 annual fee HHonors Surpass card earns 12 HHonors points per dollar. With this promotion, you'll be earning 13 and 18 HHonors points, respectively, per dollar spent with each card (although this 6 point bonus only applies to room rates, not taxes and fees).
Since there's a narrow and remarkably consistent band of value of HHonors points between 0.33 and 0.5 cents per point, this promotion is worth between 2% and 3% in additional value, bringing your total haul for a dollar spent at Hilton properties to 4.29%-6.5% and 5.95%-9% in value for each card.
Whether you anticipate taking advantage of it or not, be sure to register now, before you forget.