I really doubt that in a slow-motion shot you're going to catch the switch to 4:2:2.
The Arri D20 does not do 3K. It has a 2880 x 2160 pixel CMOS chip with a Bayer filter, meant to be recorded to HDCAM-SR (1920 x 1080 pixels) but is capable of 2K data, just like the Genesis could potentially be someday. Neither camera would do 3K. You have to switch to 4:2:2 on the HDCAM-SR deck to record slow-motion on the Arri-D20 as well.
"Sin City" was shot with a Sony F-950 and recorded to 4:4:4 HDCAM-SR (1920 x 1080 pixels). Panavision has been calling that "2K" so I guess "Sin City" can call it that too; it's close enough probably.
From the Arri site:
http://www.arri.com/news/newsletter/articles/09211103/d20.htm
But what about resolution? First of all, we have to clearly define resolution. Resolution tells us how small the smallest structures (e.g. alternating black and white stripes) are that an optical or opto-electronic system is capable of reproducing. In digital photography, there is a tendency to describe resolution in terms of the number of pixels on the chip. Depending on the technology used however, the actual pixel count of a chip does not directly correspond to the resolution the system is capable of reproducing. The D20, for instance, is designed to accurately reproduce images at HD resolution (1920 horizontal pixels). In order to achieve this goal, a Bayer mask CMOS chip of a higher pixel count is necessary.
On the Bayer mask chip itself the full number of pixels is not available for each color. For a 2880 x 2160 chip, the red channel for instance does not have a resolution corresponding to 2880 x 2160 pixels. One could assume that since every second pixel is red in every second row, we have half the resolution for red (1440 x 1080). But that is not accurate either, since for most natural images the missing color pixel values can be reconstructed very accurately, so the resolution of the red channel is somewhere between 2880 x 2160 and 1440 x 1080.
Our goal with the D20 design is to output a very high quality HD image with a resolution corresponding to 1920 horizontal pixels. In order to achieve such an image output from a Bayer mask chip we need substantially more than 1920 horizontal pixels, which is the reason the chip's pixel count (2880 x 2160) is much higher than the desired image output resolution. The raw Bayer data at 2880 x 2160 goes through the color reconstruction process to fill in the missing color information and is downscaled to a pixel count that corresponds more closely to its actual resolution. This allows the D20 to create a high definition image that looks as good as if not better than the images produced by current high definition cameras.