Dagg,
This is the site I initially found when tracking down the rules.
http://football.calsci.com/TheRules3.html
I've since found a site that actually has the official NFL rulebook(2006)
http://blogmedia.thenewstribune.com/...20RULEBOOK.pdf
Specifically, Rule 8 item g. page 82, that specific rules reads:
Impermissible
Use of
Helmet and
Facemask
( g) using any part of a player’s helmet (including the top/crown and forehead/“hairline” parts)
or facemask to butt, spear, or ram an opponent violently or unnecessarily; although such
violent or unnecessary use of the helmet and facemask is impermissible against any opponent,
game officials will give special attention in administering this rule to protecting those
players who are in virtually defenseless postures (e.g., a player in the act of or just after
throwing a pass, a receiver catching or attempting to catch a pass, a runner already in the
grasp of a tackler, a kickoff or punt returner attempting to field a kick in the air, or a player
on the ground at the end of a play). All players in virtually defenseless postures are protected
by the same prohibitions against use of the helmet and facemask that are described
in the roughing-the-passer rules (see Article 11, subsection 3 below of this
Rule 12, Section 2);