We use an observation of Maldacena to argue that gravity tree-amplitudes can be computed by inserting appropriate wave-functions into the Witten-Berkovits twistor-string for conformal gravity. This is used to motivate and partially explain the recent remarkable twistor-string-like formula of Cachazo & Skinner that expresses the complete tree-level S-matrix for N=8 supergravity in terms of an integral over rational curves in twistor space.