All the Epcot restaurants and resort restaurants plus Donalds Breakfastaursaurus - 120 days out
All MK and MGM restaurants - 60 days out (except the Fantasmic package - bookable for the moment at the beginning of the month before you go - I've just booked mine for July)
A useful link is http://pscalculator.net/ - you key in the date of departure and it gives you the date when you can book.
I also find my copy of Passporter (and boards such as this) invaluable. The Passporter gives this info too together with the opening times of all the restaurants.