I've been affected by serious banding on the Leica M9 at higher ISOs (though as low as 640, visible without any pushing), which has also been noticed by at least two people that I know of. I've written up everything I know about it here:
http://blog.markprobst.net/2012/01/24/yet-another-leica-m9-bug/It turns out the main culprit seems to be the SD card used (even when using the latest firmware), but battery charge and use of review while the buffer is busy also seem to affect it.
If any of you have had similar problems, please let me know.
