This is a burst sequence taken with a Canon 1D Mark II N in high-speed mode (8fps). You can see the bright line down the center and the dark area in the upper-right. I have several other sequences with similar artifacts.
Is my shutter failing, or is there something else going on?
Shutter count utilities at http://www.foxbat.me.uk/2.0/index.php#/1dcount and http://www.camerashuttercount.com/ both indicate a shutter count of ~79,000, though I do not know the accuracy of these sites, nor do I have an estimate of the camera's shutter count as I got the camera used with no pedigree. My understanding is that this particular model's shutter life expectancy is around 200,000 actuations and I've heard of people getting over 400,000.
UPDATE: The problem is almost certainly the shutter; see my answer below with image.