[3.5] When will helmets get the recognition they deserve?

[Matthew]

Indeed, though in first edition AD&D not wearing a helmet had a special rule that 1 in every X attacks would be directed at that unarmoured location, with results that I forget just at the moment.

[edit] Here we go:

Advanced Dungeons & Dragons, Dungeon Master's Guide (1979), p. 28.

Helmets

It is assumed that an appropriate type of head armouring will be added to the suit of armour in order to allow uniform protection of the wearer. Wearing of a "great helm" adds the appropriate weight and restricts vision to the front 60° only, but it gives the head AC 1. If a helmet is not worn, 1 blow in 6 will strike at the AC 10 head, unless the opponent is intelligent, in which case 1 blow in 2 will be aimed at the AC 10 head (d6, 1-3 = head blow).

As it goes, second edition neglected to include this rule, but, as Oracle Hunter mentions above, later introduced various "called shot" mechanics.