The original Scoville scale was indeed set up in the mid 1920s as the dilution in sugar water required for a group of trained tasters to just barely perceive (as a panel) the heat of a chile preparation. It survives as an ASTM method E-(something or other, but I do not know the number), I'll look it up.
I did hear about the HPLC method and attempts to correlate the results with those of taste panels. Of course the discovery of the other five or so capsaicinoids adds some complication. Are they all equally pungent? How are the structures different? What does it tell about the "heat" of a chile to be able to measure all five capsaicins? Do they each impart a subtly different aroma or flavor character and so are worth measuring separately? Would it be more meaningful to "chunk" them together as one unit?
[...]The Scoville rating is subjective. It was developed by Wilbur Scoville in the earlier part of this century. The process used human subjects to asess pungency. It is called the Scoville Organoleptic Test. HPLC came later because we could not find subjects who could taste habs all day without quiting their jobs and moving to Cuba ...