I had similar ideas, but I think it is technically challenging to produce such transparent displays so close to the eye (i.e. integrated in eyeglasses, for example). The eye can't focus in objects that close.
One solution could be doing some kind of retinal projection. You could adapt the optics in real time to compensate the eye's focus, and you would have a UI stamped into your normal vision at all times. The real problem is technology because, AFAIK, such systems are currently too big to be used in our day-to-day lives. ( http://en.wikipedia.org/wiki/Virtual_retinal_display )
One solution could be doing some kind of retinal projection. You could adapt the optics in real time to compensate the eye's focus, and you would have a UI stamped into your normal vision at all times. The real problem is technology because, AFAIK, such systems are currently too big to be used in our day-to-day lives. ( http://en.wikipedia.org/wiki/Virtual_retinal_display )