Dark Dragon
Explorer
... seems to be a bit vague. Or I am thinking too much about it.
From the SRD:
Some food for thought:
Assume Forbiddance was cast by a chaotic evil cleric.
So from the wording one could conclude that those with the Neutral alignment are not harmed. But that would contradict the "Alignments identical" rule...
Or gets a guy with the Neutral alignment the full dose of pain (12d6), because his alignment is different with respect to both law/chaos and good/evil?
So far I tend to interpret the spell in the latter way...

Forbiddance seals an area against all planar travel into or within it. This includes all teleportation spells (such as dimension door and teleport), plane shifting, astral travel, ethereal travel, and all summoning spells. Such effects simply fail automatically.
In addition, it damages entering creatures whose alignments are different from yours. The effect on those attempting to enter the warded area is based on their alignment relative to yours (see below). A creature inside the area when the spell is cast takes no damage unless it exits the area and attempts to reenter, at which time it is affected as normal.
Alignments identical:
No effect. The creature may enter the area freely (although not by planar travel).
Alignments different with respect to either law/chaos or good/evil:
The creature takes 6d6 points of damage. A successful Will save halves the damage, and spell resistance applies.
Alignments different with respect to both law/chaos and good/evil:
The creature takes 12d6 points of damage. A successful Will save halves the damage, and spell resistance applies.
Some food for thought:
Assume Forbiddance was cast by a chaotic evil cleric.
So from the wording one could conclude that those with the Neutral alignment are not harmed. But that would contradict the "Alignments identical" rule...
Or gets a guy with the Neutral alignment the full dose of pain (12d6), because his alignment is different with respect to both law/chaos and good/evil?
So far I tend to interpret the spell in the latter way...