By coincidence!... This very problem happened to my daughter's PC yesterday afternoon when she did the upgrade from 9.04/JJ to 9.10/KK -- Doesn't appear that she "did anything wrong" during the upgrade; Karmic just failed to recognize her monitor, even though Jaunty (and Hardy before that) handled it just fine. Who broke that code?
Anyway, the above recipe fixed things up fine. Many thanks to the community for recognizing the problem quickly and providing a very clear and usable fix! Thank you, Sharaq!