[font color=\'#000000\']Update
Just to bring you all up to date. I sent my 1Ds into Canon Factory Service in Irvine along with a sample print demonstrating the problem. They evidently thought there was a problem as well. They "replaced the CMOS, Adjusted CMOS Sensor, Shade, White Balance, Color Matrix, Cleaned VF, Mirror, and Checked all functions." (Their words, I'm not sure what some of them mean.) It even came back with a clean sensor (no dust specks at all on test image). I haven't been back to the same location, but similar shots do not show the lighter half of the frame I was seeing before.
Thanks for your suggestions.[/font]