Humidity is what causes wood to change dimension.
The summer is usually more humid and the wood swells. During the winter the humidity is lower and wood shrinks.

How much movement are we talkink about here?
1/16 inch? 1/2 inch? bigger?
Significant movment usually causes drywall to crack and seams to open. Do you have any of that?

You need to have a look in the attic and see if you can observe any obvious problems like damaged framing, cut trusses, water stains on the underside of the roof deck, etc. Anything out of the ordinary.

In the building trades we call this Truss Uplift. It is caused by moisture absorption. There is no real documented cure for this situation. However most builders and tradesman are addressing the issue with ventilation improvements in the attic spaces. Trying to minimize wood moisture absorption is the key.

I have been lucky a couple of times, trying to deal with this issue, we installed new drywall and where the ceiling meets a wall have it a little loose, to allow this flexing. We wouldn't fasten the drywall ceiling any closer that 18 inches to the wall.

I don't know how far you want to go with this divot situation, but maybe just a patch job every couple of years might be suitable for you, but I would at least try to deal with the vent/moisture issue.

Brickeye and fqp25,
Have only one ceiling crack at a joint where the kitchen meets the living room in the middle of the house. Opens to about 1/16 inch during winter and closes in summer. We've lived in the house 3 years and had it inspected top to bottom prior to purchase. Inspector said attic and roof sheathing looked very good. I went upstairs with him and he pointed out all the things to look for. Had a new roof put on in Nov 2003 (tear off down to sheathing and re-shingle + ice shield) and a new Cobra ridge vent added (original roof had a sheet metal ridge vent). I have added some additional R25 insulation in the attic the last couple of years (Chicago winters can get brutal). I plan to check my soffit vents to ensure they're clear. Anything more I can do to improve attic ventilation short of installing additional vents? Isn't it true my trusses will still absorb moisture in the summer from the humidity in the air no matter how well ventilated? Sounds like this is a something I'll just have to live with.

Unfortunately this phenomenon is not well understood. Some people relate it to younger generation wood used for todays lumber supply. Being in the Chicago area(GO BEARS) you, and I have changing cold temperatures in our attics. Some people have actually tried insulating the bottom chord of the truss itself. It is literally too early to decide if one fix is the ultimate solution. I have stated I hang ceiling drywall with special hangers which doesn't allow a fastener within 16-18 inches of a wall. This seems to have worked for the time being.

The most recent attempt to hide the crack is to secure some kind of molding to the ceiling, and not the wall. This should conceal the crack while the truss bows upward. You really shouldn't worry about structural effects, so far it just seems to be an annoyance.