According to a representative we spoke with at Hamilton Hodell, the agency that represents Emma Thompson, there will be no appearance by Sibyll Trelawney in Harry Potter and the Half-Blood Prince.

You may recall in the book that it was Trelawney who indirectly informed Harry of two important facts, 1) Draco was 'celebrating' in the Room of Requirement and 2) Snape was the one who overheard her telling Dumbledore about Harry/Voldemort's prophecy.