According to the 2012 DDP brochure, TS credits can be redeemed for a CS meal. It is a one-for one trade, you don't get 2 CS meals for one TS credit. In the past it has been at the discretion of the CS restaurant but now the brochure specifically lists this as an option.
This is on the regular dining plan. Deluxe dining credits are fully interchangeable for either TS or CS.