What parts of the regular Linux graphics driver architecture is used in Android? For example, could the Nouveau driver be used and provide hardware accelerated drawing when Android is compiled for x86?
Noveau is a driver for the X.org graphics system so it definitely couldn't be used verbatim. The one part that I'd expect to possibly be useful would be the code that actually talks to the hardware. The same is true for any other X.org driver.

But in Noveau's case, most of the work is in reverse-engineering the hardware interface as Nvidia refuses to release the documentation. This work could definitely be reused in a hypothetical Android Nvidia driver.