Studios are digitizing their films at 4k lines of resolution now, because there are digitizing houses for this and 4k is supposed to equal the resolution that film will provide. I suspect some of the high-definition content coming out will be off of those files.
If those studies (which I can't reference now) are correct in that 4k of lines is equal to film, then 4k should theoretically be as high-def as we need to go. At least until people start making movies with something more sensitive than film, or 3-D.
