The only cast member that has been 'officially' confirmed is Kyle MacLachlan, plenty have hinted at being involved and Sheryl Lee, Madchen Amick, Dana Ashbrook and Peggy Lipton have all been spotted in Washington. The only ones I can't see returning (apart from those that have passed away and minor characters) are Lara Flynn Boyle, James Marshall and Michael Ontkean, the latter has no interest, the first two apparently haven't been asked. There are rumours Moira Kelly is returning as Donna and Ray Wise is said to be involved.
I'm off to the Twin Peaks festival early next month, Sheryl Lee, Dana Ashbrook and Madchen Amick were all supposed to be in attendance, the first two have pulled out due to 'filming commitments' so that's a pretty big hint, Madchen Amick is still attending though.