Compare four situations: 1) Space flight in 1950, 2) Heavier than air flight in 1900, 3) Heavier than air flight in 1200, 4) Drexler nanotech today. Since the science was lacking - no one had ever built the relevant technology, we have to approach the question indirectly. The best questions to ask are:

A) Does something like X already exist? For 2) and 3) yes (birds flying using mechanical force), for 4) yes (enzymes doing similar roles). For 1), no.

B) If something like X exists, do we understand it? For 2), yes, for 4), partially, for the other, not at all.

C) Do current technologies exist that can approach X without new conceptual ideas? 1) yes, (rockets), 2) yes (lifting surfaces and models of gliders), 3) no, 4) probably (larger scale nano and medical manipulations of enzymes, proteins and retro-viruses).

So I'd put Drexler's nanotechnology in with flight in 1900 - the signs are that something like what he describes (certainly not exactly as he describes) will be coming in the forseable future.

(Unfortunately, I'd put AI down with flight in 1200 - intelligence exists, but we don't understand it to any real extent, and current technologies are not approaching proper intelligence; they need new conceptual ideas)