I have been chasing this problem on my Nissan 1995 pickup for several months now with no obvious success until today (hopefully). I have replaced the starter twice (the first time was for a bad starter clutch), the battery twice, the clutch inhibit relay several times, and most of the wiring which connects the relay to the starter.
The problem has been intermittent with long periods of not showing its ugly head to times when it crops up every day. Recently it occurred every day, usually in the morning on the way to work. When I turned the key I either heard a click or a stutter from the clutch inhibit relay, or in some cases nothing. Using a multimeter I would trace the fault back a ways until the starter began acting normally and thus preventing me from continuing to chase the problem.
Today I managed to isolate the problem to the ignition switch. This switch has two major components, one is the steering wheel lock and the portion where the key is inserted, and the other is the electrical switch. The mechanical linkage between the two components had become slightly worn and loose. The steering wheel lock portion limits key rotation to prevent the electrical switch from rotating beyond the start position. The wear and loose allowed the electrical portion of the switch to rotate far enough that the limit on key rotation prevented the key from being able to activate the starter. I simply rotated the electrical portion back and tightened the screw holding it and viola, the starter engaged reliably. Before I tightened the screw I was able to keep the key from starting the car by rotating the switch one way and force it to start the car by rotating it the other way.
I think the switch will eventually get loose again, so this is probably not a permanent fix. Since Schucks wants $160 + shipping for a new ignition switch, I will probably simply install a button on the dash which I can push to engage the starter.