It would appear this is published on the Marriott website, for example;
Originally Posted by https://www.marriott.com/rewards/moreRewardDetail.mi?marrRewardCode=390&points=20,0 00
Note: Beginning January 2012, the Dinner-for-Two reward* will be replaced with the Dining GiftCard. This new card will be available in denominations of $100 and $200, and have no expiration date. It will maintain its balance until depleted and be accepted at Marriott restaurants worldwide.
* Rewards redeemed before December 31, 2011, will be honored until the expiration date.