intense banding in some SD1 photos

I have a question for those of you who either own an SD1 or have had a chance to work with one, in many photos I see really bad banding in the smooth out of focus areas such as the one posted below. Is this a common problem with the SD1 or just really bad processing or severe underexposure? And have any of you had this problem? In my on going debate whether to buy an SD1 this is my main concern, as a lot of my images have this kind of background and banding like this would ruin every single one of them. I can't help but feel that Sigma, in order to play the megapixel race, compromised IQ in exchange--not a good decision IMO.