Of course, tools are not invisible in themselves, but as part of a context of use. But good tools enhance invisibility. I think the value of invisibility is generally understood. Unfortunately, our common metaphors for computer interaction lead us away from the invisible tool, and towards making the tool the center of attention. The idea, as near as I can tell, is that people already spend hours a week at home watching television, so clearly television is attractive, and we want our computer interfaces to be attractive, so let's put TV into them.
Attractiveness is the opposite of invisible.
Take intelligent agents. The idea, as near as I can tell, is that the ideal computer should be like a human being, only more obedient. Anything so insidiously appealing should immediately give pause. Further, it takes a lot of time and attention to build and maintain a smoothly running team of people, even a pair of people. But magic is about psychology and salesmanship, and I believe a dangerous model for good design and productive technology.