It looks more like a bad case of scale to me, Ben, though that doesn't exclude spider mites, too. I'm going by the visible symptoms and the visible scale in the last picture. Unfortunately, none of the pictures are really close enough, or have enough resolution, to show spider mite symptoms. The evident web in the last picture could be spider mites, or it could just be an ordinary spider web. It wouldn't hurt to wash the bushes off well to discourage mites anyway.

Sorry I cannot tell you what is attacking your plant but the link I have given describes a lot of the diseases and problems with pittisporums. It might help you to look at it. http://plantpath.ifas.ufl.edu/media/plantpathifasufledu/factsheets/pp0029.pdf

It looks like Pittosporum tobira 'Nanum' to me. As it seems to be the only specimen that is dying and it does look quite woody, then it could be just that it has completed its lifespan and the adjacent pests which look like spiders (you will know better than I concerning your indigenous pests) but may not have had any affect on the plant.

I know how it feels...

Thanks all! Okay, so I did more investigating. Not scale, can't find any scale anywhere. I think what you are seeing Tug is the bark texture that is blurry. There is webbing all over all these hedges but it isn't the normal spider mite webbing - it is larger and thicker. I also can't find any mites. Some black "dust" is found under some of the leaves - could that be them, or droppings of some sort?
It is all very woody. There is also an adjacent area that has similar death so it can't be that singular hedge. The hedge is 12 years old - that seems short lived?

Could it be disease? Or fungal? It has been unusually wet here in San Diego. There is a large area of this hedge and it hasn't put forth new growth yet, and the little buds are quite yellow. Overall, there is not much vigor or health.

Would you cut it back? It is a centerpiece with nice low hedged design and it would be a shame to lose it if whatever this is spreads.

Google Rhizoctonia Aerial Blight. It is mentioned in the first link I gave you but other links describe the 'web like' problem in greater detail. I thought the leaf rolling might indicate this disease too. The problem seems to be worse in wet situations.

Ben, I thought I saw a few red scales on the leaves in the foreground of the last picture, and I know from experience that for every scale that you see on the top side of the leaf, there are a hundred that you don't see, attached to the twigs and leaf undersides. "Black powder" on the leaf undersides is often sooty mold, which is also a symptom of scale infestation. When there are enough scales feeding on the plant, random branches will wilt and die, sometimes eventually killing the whole plant.
I could be wrong, though, and the "scale" that I think I see is only stray Eucalyptus flower caps! :)