They don't call the Kinect port the "NUI Port" for the same reason they don't call the HDMI port the "Metro Interface Port". Kinect is the hardware that makes the Natural User Interface possible. Kinect IS the simple microphone (array). It is a simple webcam. It is a depth sensor. They're not going to include simple room mics for the same reason they dropped support for the Xbox Vision camera, and never supported voice commands using a 360 headset. Their vision of a natural interface requires Kinect. It's that simple. Anyone who thinks Kinect won't come standard is simply kidding themselves.
Actually bkilian said Kinect CANNOT be integrated into the Xbox chassis. Audio spacing, positioning, etc. Think about the current Kinect sensor and where you place it.
So it's literally impossible for it to be non removable.
I do think Kinect will be standard out of the box, but I dont think it's even possible for MS to not have the option to ship a non-kinect SKU down the line (although I'm not even sure this would be a good idea).
As I've stated, the audio chips have voice recognition for kinect built in, so thats at least some built in expense to the main box for kinect.
And I know thats really what this is all about, a lot of people want Kinect built in so it will in theory weaken the main hardware so PS4 can be more powerful.
But I think it's a case of be careful what you wish for, there's a non insignificant chance Kinect will sell a lot of boxes, or that even with Kinect packed in Durango will match Orbis pretty well, leaving Kinect only as an additional selling point, not an anchor. The same goes for all the OS stuff supposedly planned.
If you end up with Durango being comparable to Orbis or within ~10%, but it has Kinect and whatever OS stuff MS has planned for the same price, I suspect that overall package will sell a lot better than a possibly slightly more powerful Orbis, at least in MS friendly territories.