noise on the 5d ii

Hi,

I've recently purchased the 5d mark ii and had understood that it would go to very high iso before generating noticeable noise in a picture.

I took some test pictures to see how it performed.  The attached image was taken at 640 iso, shutter speed 320 and aperture of 11 - yet when viewed at 100%, the sky shows much more noise than I was expecting (not sure whether you can zoom in on this image to see that though??)

Is this typical or an issue with my camera?

Thanks

Laura