I am looking at the official NFL rules from 2007.
Rule 3, Section 38
A Touchdown is the situation in which any part of the ball, legally in possession of a
player inbounds, is on, above, or behind the opponent’s goal line (plane), provided it is
not a touchback (11-2).
Rule 11, Section 2 Touchdown
Article 1 It is a touchdown (3-38):
(a) the ball is on, above, or behind the plane of the opponent’s goal line and is in possession
of a runner who has advanced from the field of play; or
(b) a ball in possession of an airborne runner is on, above, or behind the plane of the
goal line, and some part of the ball was passed over or inside the pylon; or
© a ball in player possession touches the pylon, provided that no part of the player’s
body, except his hands or feet, struck the ground before the ball touched the pylon; or
(d) Any player who is legally inbounds catches or recovers a loose ball (3-2-3) on or behind
the opponent’s goal line; or
(e) The Referee awards a touchdown to a team that has been denied one by a palpably
unfair act.
I DO NOT see the section that was listed earlier:
"Should a receiver make a legal catch of the ball with both feet in bounds in the end zone, a touchdown shall be awarded even if no part of the ball was deemed to break the plane of the goal line while in possession of the receiving player."
I suppose that there may be a broader version of the rules than I have, but I have not seen a link to the legal catch citation above.