Just checked Foursquare and several restaurants in my area are repeating the Foursquare/American Express promotion from earlier in the month. Check in to a business displaying the deal on Foursquare, unlock the promotion, and if you spend $10 there within 24 hours on your registered American Express card, you get a $10 statement credit. TIP: If you aren't heading out to eat, use it at a participating bakery or bagel shop.

From Foursquare.com:
Shop Small to support small businesses! Load this offer to your synced Amex Card: Spend $10 or more in one purchase at a participating small business and get a one-time $10 statement credit. Statement credit fulfilled by Amex and processed within 3-5 business days. Purchase must be made within 24 hours of offer load. Limit one $10 credit across all participating merchants.

UPDATES: More details below, and thanks for all the reps :-) :

To find deals near you, launch the Foursquare app from your phone. Click on Explore, then click on Specials. It will list up to 30 specials within 10 miles. Look for the ones marked with the AmEx logo.

Follow these steps and you won't have a problem:
1. Sync your American Express card to your Foursquare account [S=HERE]https://sync.americanexpress.com/[/S].
2. Check into the merchant using the Foursquare app.
3. Unlock the special.
4. You are invited to load the special onto your card. Don't skip this step.
5. Spend $10 or more on your synced card within 24 hours.

You must spend using your registered AMEX card at the merchant within 24 hours after checking in via Foursquare and activating the deal.

Ends Saturday 12/31. Those who checked in and activated by 12/31 are reporting that they could still use the offer for another 24 hours.

If you have multiple AmEx cards and want to try double dipping, you'll need to sync each of the cards to a unique Foursquare account and check into the restaurant with the correct account. This works whether you have two different AmEx cards under your name, or you have multiple "additional" cards on a single account. They just have to be unique AmEx numbers and unique FourSquare accounts.