My position:
1. Improved Unarmed Strikes are not natural weapons. They are merely treated as natural weapons in some specific circumstances.
2. A feat is not an effect. A feat does not appear to be able to have an effect. A feat has a benefit. A spell can have an effect. Acid has an effect. Smoke has an effect. Ice has an effect. Lava has an effect. Supernatural abilites can have an effect. A feat is a "special capability". Nowhere does it state (TMK) that a feat is or has an effect. I cannot find a "feat effect" like I can a "spell effects" in the books.
3. No, monks cannot take INA according to RAW.
4. In any case, I don't think it is unbalancing.
Note: I view this as a literal interpretation of RAW. I can understand the view that feats affect the game, hence, they have an effect.
I'm hung up on the phrase:
"A monk’s unarmed strike is treated both as a manufactured weapon and a natural weapon for the purpose of spells and effects that enhance or improve either manufactured weapons or natural weapons."
If they had wanted it to be a natural weapon, they should have just said so. But they did not, probably because it does not follow the rules of natural weapons.
I think this statement was originally here solely to allow Magic Fang and Magic Weapon (and similar spell and spell-like effects) to be cast on Monk's hands.
I also do not think that the Weapon Finesse feat is an effect that can be applied to Monks Improved Unarmed Strikes, even though it improves manufactured weapons and improves natural weapons. Unlike Weapon Focus and Weapon Specialization (and the Greater versions of these), Weapon Finesse does not explicitly include unarmed strikes. Natural weapons are listed in this feat as light weapons, but unarmed strikes are mysteriously silent here. Unarmed strikes are also not listed in the weapons table as light weapons (like natural weapons are considered), rather they are listed as unarmed attacks.
The definition of Natural Weapons does not appear to apply to Monks. For example, Monks DO receive additional attacks from a high base attack bonus. Natural Weapons do not. Some of the text of Natural Weapons is similar, but much of it is different and does not apply to the Monks Unarmed Attacks, hence, they are not Natural Weapons. For example, Improved Unarmed Strike is not listed anywhere as a Natural Weapon type.
"Natural Weapons: Natural weapons are weapons that are physically a part of a creature. A creature making a melee attack with a natural weapon is considered armed and does not provoke attacks of opportunity. Likewise, it threatens any space it can reach. Creatures do not receive additional attacks from a high base attack bonus when using natural weapons. The number of attacks a creature can make with its natural weapons depends on the type of the attack—generally, a creature can make one bite attack, one attack per claw or tentacle, one gore attack, one sting attack, or one slam attack (although Large creatures with arms or arm-like limbs can make a slam attack with each arm). Refer to the individual monster descriptions.
Unless otherwise noted, a natural weapon threatens a critical hit on a natural attack roll of 20.
When a creature has more than one natural weapon, one of them (or sometimes a pair or set of them) is the primary weapon. All the creature’s remaining natural weapons are secondary.
The primary weapon is given in the creature’s Attack entry, and the primary weapon or weapons is given first in the creature’s Full Attack entry. A creature’s primary natural weapon is its most effective natural attack, usually by virtue of the creature’s physiology, training, or innate talent with the weapon. An attack with a primary natural weapon uses the creature’s full attack bonus. Attacks with secondary natural weapons are less effective and are made with a –5 penalty on the attack roll, no matter how many there are. (Creatures with the Multiattack feat take only a –2 penalty on secondary attacks.) This penalty applies even when the creature makes a single attack with the secondary weapon as part of the attack action or as an attack of opportunity.
Natural weapons have types just as other weapons do. The most common are summarized below.
Bite: The creature attacks with its mouth, dealing piercing, slashing, and bludgeoning damage.
Claw or Talon: The creature rips with a sharp appendage, dealing piercing and slashing damage.
Gore: The creature spears the opponent with an antler, horn, or similar appendage, dealing piercing damage.
Slap or Slam: The creature batters opponents with an appendage, dealing bludgeoning damage.
Sting: The creature stabs with a stinger, dealing piercing damage. Sting attacks usually deal damage from poison in addition to hit point damage.
Tentacle: The creature flails at opponents with a powerful tentacle, dealing bludgeoning (and sometimes slashing) damage. "
Finally, the FAQ argument is just as silly in this thread as in every other thread.