In the old days I'd say 21-23lbs was the lighest you'd get for a good bike.

I think stuff was actually lighter in the 70s. I was weighing some old derailleurs about a month ago when I was going through my parts and all the 70s derailleurs I had were always lighter than my 80s one, guess it has to do with the indexing? I dont know! The same was true with brake calipers, the single pivot ones were lighter than the dual pivots.

You could probably take an old normal sized good quality lugged steel frame from the 70s or 80s and go to town on it with new good parts - dura ace/ultegra or record/chorus for you italians and a good light weight wheel set and have a 18 pound bike in the end... I can't see any other way to get it any lighter without compromising safety!