Doc talk:2.6/Manual/Modifiers/Deform/Displace
2011-05-01: It seems that when we set the displace modifier's coordinates to be based on a 2nd object, moving the original object also changes the coordinates of the mapping. This surprised me and I raised a defect (27248). This was returned as closed claiming that it was as designed. I have updated the article page with this information - Kolban.