In all too many new houses there is a seasonal problem where in the winter the ceiling actually lifts itself right off the wall in the centre of the house, and then settles down after the heating season. Such is the case with Mike in Calgary.
This is called truss up-lift and it is caused by different expansion and contraction of the top and bottom cords of some truss configurations. There is no way to predict which truss will cause this problem because it is actually related to fiber stress in the original tree. We can't stop it from moving either because it is too powerful to tie down. To avoid it we can use more flexible truss designs, like raised heel trusses, or let the ceiling float to hide the movement. Letting the ceiling float means attaching ceiling panels to the central wall but not putting screws into the ceiling for 18 inches back. The truss can arc up and down, the ceiling will flex but you will never see it inside.
But Mike doesn't want to take the ceiling down and start over with new drywall. So the next best way to hide the crack that you can't stop, is to put a large crown molding on the wall, drop it about 1 or 2 inches below the ceiling to leave a decorative shadow line. Now the crack is not longer visible and the joint between the wall and the ceiling is a wide shadow line formed by the molding. You could even put rope lighting back behind the molding and have a beautiful light sneaking onto the ceiling. So what started as an ugly crack justifies a great decoration.