I didn't think of insurance. You're right that that would be a good avenue to investigate.
The roof wasn't really old, though--only a year and a half, installed summer of 1999. My architect friend left me with the impression (after he looked at our dormer) that the former owners didn't do it very well (& I'm almost positive they did it themselves)--and where he pointed to potential problems is where the plaster is now seperating (knee wall from ceiling). Thing is, where the roof is sagging is on the very steep side of the attic, not the flat roof area atop the dormer. I had the flat roof area shovelled and didn't need to have the steep side shovelled. So it would be hard to point to my upkeep as a problem. I think the thing to do in my situation is 1)talk to insurance and 2)have a structural engineer come in and measure. My mom: she didn't like Georgia Pacific at all, it was a boy's club through and through, so I think she was relatively objective when she said that often the problems she saw weren't really the fault of the shingles. But if you knew my mom, you would find the image of her climbing onto roofs entertaining in and of itself. She's a spritely little gray-haired woman. :-)
thanks for input. I think the insurance avenue is a good one.