There's another thread discussing this issue here:
that includes some pretty good images of the problem. Although the thread starts to wander, and at times breaks up into bouts of spasmodic defensiveness. (I honestly don't understand why technical discussion should provoke such defensiveness in some folks. If we talk about these things, they'll get fixed - or at least we'll generate workarounds. Everybody loves the camera - just chill out.)
I'm going to have to weigh in on the "yes, I've seen it" side. It's definitely noticeable - particularly with tungsten-balanced lighting, mostly in the blue, some in the red, and in areas of low detail (which is, paradoxically, high detail - due to noise... i.e. your eye is going to pick out noise/grain more easily over a smooth surface). And it's definitely visible in motion as well, contrary to what some have suggested.
The thing that really gets me is the *size*. Check out post #8 in the thread above, in the upper-right corner over the blurry background. I'd think that sensor noise would be pretty high frequency - it surprises me that the codec responds with artifacts of that size. Then again, I stopped taking math classes as soon as I was allowed to.
In motion (RGB), it appears as a subtle crawling/pulsing in the luminance of the image. And I think this is the crux of the problem... i.e. film can get plenty grainy in the blue channel as well - but it's an organic effect that we're all used to. Compression / video noise artifacts just look weird.
I'm not sure either of the workarounds already suggested are ideal: 1) blur the blue channel (will piss off the compositors), 2) shoot everything daylight balanced (will piss off DP's and/or producers), 3) wait for uncompressed RAW output.
But I'm hoping with further discussion, we'll come up with something. (I will say, a selection of data rates/compression ratios in the future would be, in the vernacular, "dopesauce". Be great to just crank the rate when necessary.)