I don't have this textbook, and unfortunately Google Books won't show me pg. 269 in preview. I have read Wald's monograph "Quantum Fields in Curved Spacetime and Black Hole Thermodynamics". From what I read there, it seems to me that one could just as easily interpret what is happening with Hawking radiation as "incoming quantum field modes corresponding to a vacuum state scatter off the spacetime curvature of the hole in such a way as to transfer energy from the hole's curvature to quantum field modes corresponding to outgoing particles". No "negative energy" anywhere, just a perfectly ordinary scattering process.See the standard textbook Birrel Davies "Quantum Fields in Curved Space", the un-numbered equation at page 269. Is it an interpretation or a prediction?