Actually it's the skin tones that are more of a problem with the 645D at high ISO.
While the Pentax does a very good job with high ISO as far as luminance noise goes
it's not good on skin tones at ISO 1600.
Being a portrait photographer it's the skin tones I look at and due to the type if lighting I use
I also look at the color in the areas of skin tone that fall into the shadows.
Having taken a look at the Pentax 645D I decided against mainly due to the 44x33 sized sensor,
but also for the high ISO skin tones.
They are more ashy and slight green patches appear in the gradations of the skin.
Anyway you can see what I am talking about from the same DPreview test comparison tool
that was shown above.
And here is the Pentax image laid over the Nikon image
The Nikon has more color in the skin tones and if you drop the saturation to that of the Pentax
you will also reduce chroma noise.
Pentax image laid over the Nikon image with saturation corrected to match.
On top of this in a low light situation but with more contrast which is often the case in
night scenes the dynamic range of the camera combined with ISO perfomace comes into play.
The D800 (CMOS) has a one and a half stop dynamic range advantage.
Now that said the Pentax 645D is a lovely camera with many features that distinguish it from the other MFDB cameras.
The ergonomics are really good and what is most important it comes from a company that has significant CMOS experience
and they are likely to be the ones to make the jump over to CMOS. They also already have a good relationship
with CMOS manufacturers and already use Sony sensors.
A full frame CMOS sensor in a Pentax 645D with Pentax pricing and image stabilization
in the right lenses and I think I might get one.