A couple years ago when I remodeled the baths in this 1917 house, I changed the leaking cast iron stack to PVC from the basement to the roof vent. I also spent a few hundred bucks on specialized sound deadening matting that is supposedly designed specifically to quiet the sound of water flowing through PVC. Unfortunately the main waste runs down through the living room wall behind the sofa, and it's still loud every time someone upstairs flushes.

Now I'm reading about the new This Old House project, and one of the plumbing "upgrades" was replacing the PVC with new cast iron "for much quieter drains." This is the first I've heard of cast iron being used in new (or remodeling) applications...are there new ways of cutting/connecting it that are easier then the old ways?

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Not a plumber by trade but a fierce DIYer

cast iron pipe manufacturers used to call their products "the quiet pipe". (they still may?)
cast iron pipe and PVC have virtually the same OD.
so transitioning from one to another is simple.
to accomplish this, no hub clamps are used all the time.

Any room for insulation between the wall and pipe?

My 2 cents on cast iron....

In my neck of the woods(southern California) cast iron has and still is installed as a up grade in residential construction because it is as Packy stated "the quiet pipe". It does make a lot of difference. High end homes almost always have cast iron.

They even do it in some tract houses - usually just the vertical drops.

Commercial buildings typically have cast iron. I think you can now use ABS/PVC...would have to confirm this. I don't do new construction, so I'm not always up on the codes.

While cast iron is great at suppressing noise it does have a tendency to crack/split.

I generally (but not always) see this in commercial buildings where the pipe has not been used for several years (empty suite), people move in, start using fixtures and leaks show up.

Some people say this only happens to imported pipe. I've seen in both - domestic and imported. Most of the time it is the pipe but I have also seen fittings crack/split.Same - domestic and imported.

Normally older pipes- 30 plus years. I know in some areas that is not old.

Off hand I don't recall running across any ABS/PVC pipe that just cracked/split like cast iron. But I have not been doing this as long as HJ. In time I'm sure I will see some.....

I have seen some ABS fittings from the late 60's into the 70's that are a day old guacamole color that almost always have cracks where the hub joins the body of the fitting. The fittings where made by GSR Sloane or something to that effect. They have since gone out of business.

As Packy stated it is a piece of cake to join ABS/PVC to cast iron with a no hub coupling. Has been for years.

My 2 cents.