If you are talking about desktop operating systems the answer is never. In the server room and on embedded devices it's already there.

Edit: I shouldn't say never. If someone takes a Unix system, stops the design by comittee, and manages to solve all the bugs that normal people (grandma) would run into and be unable to solve, while developing grandma usable software that can compete with the offerings available on windows it's possible. See MacOS X as an example of how to do this correctly. Until then it will remain the tool or server admins, developers and hobbyists, and never see mainstream acceptance.