Hi Lisa,
Last year, T-Rex Cafe was on the Disney Dining Plan.
Unfortunately, the Downtown Disney restaurants are usually updated on the next year's dining plan very late in the year. If you look at the
2013 plan, you'll see that the Downtown Disney restaurant listings are scarce right now. However, as non-Disney owned restaurants sign on to the next year's Disney Dining Plan, they will be added to the list.
Since the restaurant, T-Rex Cafe, was on the plan last year, it's likely to be there in 2013 as well, however there's no guarantee.
I would advise making a reservation and checking out the link above closer to your vacation date. Thanks!!