I just wonder. I tried an impromptu experiment soon after I got my 1Ds because the uneven horizon was the first thing I noticed when I reviewed the pictures. When I re-read my question I certainly sounded like a newbie, but I've been shooting a long time and never had a problem with uneven horizons/horizontals until I picked up the 1Ds. The experiment was this:
I set my camera on a tripod and got it set level (camera back level with the plane of the table), aimed down at a table where I placed a sheet of graph paper. I set a horizontal line of the graph paper just touching along the bottom edge of the viewfinder and marked each corner on the graph paper that I could see through the viewfinder with a black pen. Then I took a shot. Interestingly enough, the graph paper lines on the photo were not level. I measured with a level the back of the camera and the table top. My measuring mark was the inside edge of the left hand line on the level guage (to be consistent between camera and table top). Granted, this is not perfect, I should have used a copy stand or similar device. As I said, this was an impromptu set up, even though it took a while to put together.
I'm going to do a different test using graph paper taped to the wall rather than on a table top. The same caveats apply to this test than to the one described above, but it's the best I can do with my meager "testing facilities"