As Dodgeman described, it's due to "truss lift." There's nothing structural that you should do. Tryng to connect the truss to the top plate of the wall could actually cause more problems.
With truss construction, when the drywall was hung it could have been floated within 24" of the innterior walls. That way when the truss' bottom chord lifts, the drywall, which is not screwed to the truss near the interior wall, simply flexes instead of cracking.
A cosmetic fix: Crown molding. Use a backer box or inverted "L" to cradle the crown, and attach the "L" and crown assembly to the trusses and not the walls. This way, when the truss lifts, the crown will move with the truss, sliding up the wall. The crack will never be seen. When the truss drops, the crown drops.