Just been over to dpreview and fredmiranda. The upshot is: nobody knows!!!!
Bojan: your theory is sound except that in many, but not all, cases banding shows up in the same spots every time. Roughly in order of acceptance by those on these forums:
Theory A) "Something" to do with the way the firmware scans/processes the image.
TheoryB) Masked pixels being read onto the image.
TheoryC) RF interference
Theory D) Quality control. These are mass produced cameras and as such thre's bound to be some variance.
Thery E) Some combination of the above
In any case the banding in your shots is quite exceptional. If it's still under warranty, send it back alon wit prints and full EXIF details to Canon.
Good luck!
Peter