Interesting sky in DP1 image (noise, banding?)...

I honestly haven't shot much with the DP1 since I got it about 2 months ago. I am becoming more accustomed to it and enjoy the camera and its output. However, on a few images today, this one in particular, I noticed something going on with the sky.

If this has been discussed, I apologize. I don't remember reading other's remarks on this - but then again, I spend most of my time on Olympus SLR talk.

What is going on here? I assume everyone else can see the odd semi-circle noise/banding in the sky here... Is this common, if so why? Shot was taken mid afternoon, sun behind and to the left. Processed RAW in SPP 3.2, no sharpening applied, a little highlight recovery, and levels adjusted in PSE6.

Any thoughts, input?

Thank you.

--
Joe - Philadelphia, Pa
http://joecorvaia.zenfolio.com/