As the diagram on the right shows, the concept of Nikon CLS is broken down into a few areas of competency and I have been happily exploring each of this area (except wireless commander mode) which the Nikon SB-400 does not support.
But that aside, it works 90% of the time and its superb and bargain to use one, as opposed to the more advanced SB800/900. FV lock was especially useful while the Auto FP high-speed sync allows beautiful night shots even at high shutter speed.