> How long until we're seeing a "computer" for sale from a major
> manufacturer which is fully componentized for modular use?

As mentioned in another comment, Always Innovating already offer modular
components in their Smart Book. http://alwaysinnovating.com/
I guess they may not be a "major manufacturer".

Always Innovating Smart Book already had detachable keyboard.
http://alwaysinnovating.com/