Normally an updated brochure will come out around the start of the year with those who have signed back up.
Typically, you can expect every restaurant to sign back up. In the history of the dining plan(s) only 1 restaurant did not sign back up, and that was Fulton's after the first year of the plan. For 2012, they came back aboard along with several new friends (TRex, RFC, etc). It's relatively safe to assume that all restaurants that were part in 2012 will be signing on for 2013. Of course, there's always a chance that they don't, but it's very slim.
So, I'd say to go ahead and plan as though the 2012s will be on the 2013 plan and just keep an eye out over the month of Jan to double/triple check that they are back.