iPhone with standard cable to M8 iDevice version = "Official" method
IPhone with Camera Connection Kit or Lightning-to-USB + iOS 7 + another USB cable to an outboard DAC like the M8 = workaround. Only appears to work with some DACs and at specific sample rates in some cases. As to correct bit depths, sample rates, clean audio, considering the app used -- not sure if anyone can answer the issue of compatibility that clearly. It is a definite "try it and see" situation. For me, I'm no longer tracking this issue for the M8 because I have some older iDevices that need the iDevice version anyway.
I also agree that iPhone-with-CCK might not work forever. It appeared without any announcement and might go away with no fanfare as well in a future OS update.