Amtrak Guest Rewards is offering free trips in addition to standard points earning for travel between January 4 and February 29. (Registration required separately for the Acela and for the Northeast Regional offer, for which you’ll need to be logged into your account.
Three Acela trips earns a free Acela trip, and two Northeast Regional trips earns a free Northeast Regional, with free travel having to be taken between July 4 through August 30, 2012. Acela redemptions are subject to time restrictions on weekdays.
The offer on the Amtrak Guest Rewards website only references the Acela deal, but Amtrak Guest Rewards representatives have communicated online that it applies even more generously to Northeast Regional trains.
(Post updated thanks to commenter Cameron)