Passing of file descriptors over the X protocol socket is needed if using the DMA-BUF interfaces for passing DRI objects between the client and server, which is one of the main features of DRI-Next/DRI3. The DRI3 implementation details are outlined in this article.
Keith Packard has outlined his FD passing adventures on his blog. Next on his agenda for updating the Direct Rendering Infrastructure is creating a simple FD passing extension that works for passing descriptors back and forth to the X Server. Once that is done, the rest of "DRI-Next" should come together in a straightforward manner.