I don't think I have any drives here that do obey that, Jeff. I got curious about this, oh, maybe a year back when this discussion first took place on another list, and wrote a test gizmo that copied a large file, then slept for 1 second and issued a sync command. No drive led activity until the usual 5 second delay of the filesystem had expired. To me, that indicated that the sync command was being returned as completed without error and I had my shell prompt back long before the drives leds came on. Admittedly that may not be a 100% valid test, but I really did expect to see the leds come on as the sync command was executed.
Again, probably not a valid test of the sync command, but thats the evidence I have. I do not believe it works here, with any of the 5 drives currently spinning in these two boxes.