I am happy to announce that the spot problem appears to be solved. Last night I installed version 173.14.09 of the nVidia driver (hint for Ubuntu users: enable the "hardy-proposed" repository, and install nvidia-glx-new-envy--unless you have an older nVidia card, of course), and this morning beneath the swaying palm trees of the Tampa Bay sim I took a picture, a crop of which you see here. With hopeful fingers on the trackball I opened it with GIMP and zoomed to 200%... and found no spots!
So... I can return to taking pictures untroubled by the fear that a lovely subject and what efforts I can muster to compose and light will be wrecked. Whee!