I noticed my copy of the 8-15mm fisheye is not centered. The distance between the image circle and the left side of the image is shorter then the right side. Top and bottom is correct.
Can other owners check their copy?
I'd expect an L lens to be flawless in build. So if others dont have this, I'm exchanging it at the shop.
as pointed out, all lenses do that, one usually just does not see it. As the manual states, these are interchangeable lenses, the center of the lens may not exactly coincide with the center of the sensor.
In fact, if you mount a lens, before it locks into place, you will notice that you can shift it around in the mount quite a bit.
If you do vignetting tests with other lenses, you will see asymmetric hard vignetting also with other lenses. http://photonius.wikispaces.com/Vignetting