No one really knows. They might have prepared a special burial and worshipping spot ofr her because she taught the Na'vi more about the human world and died fighting for them. No one is sure though because Sigourney Weaver ahs signed on for AVATAR 2, and she will portray the same character, Grace. Not too many fans are surprised of her return because of her acting talents, but even though she died in Jake's arms saying, "I am with Eywa now. She's real", people ponder at whether or not being with Eywa means death.