> Current Intel CPUs have a very small GPU built on to the die of the CPU. By buying the CPU, you're paying for an Intel GPU anyway.

I was not aware of this fact.

> the Intel GPU to be connected to the display hardware and be used most of the time...the NVIDIA GPU is enabled on the fly...

I did mention these aspects of Optimus.

> rendering is done on the NVIDIA GPU, and the final result is sent in some way to the Intel GPU, which can then use its actual display connections to put something on the LCD.

I guess I missed the point that the architecture is like this:

nVidia <-> Intel <-> Display

instead of like this:

Display <-> nVidia

Display <-> Intel

I was a little fuzzy on this point myself, so I appreciate the clarification!

> the infrastructure for the actual sharing of the rendered output didn't really exist until dmabuf appeared

I'd certainly believe that the current approach to the nVidia driver was enabled by dmabuf. But Bumblebee shows it's possible to use Optimus on Linux without that particular kernel feature.