Caliban said:
If you use a double weapon for two-weapon fighting style, it is treated as a one-handed weapon and a light weapon for all purposes, included Power Attack.
That's debatable (and debated).
It's certainly treated as a one-handed weapon and a light weapon
for the purposes of penalties related to two-weapon fighting.
"A character can fight with both ends of a double weapon as if fighting with two weapons, but he or she incurs all the normal attack penalties associated with two-weapon combat, just as though the character were wielding a one-handed weapon and a light weapon."
There's also a note under Power Attack:
POWER ATTACK [GENERAL]
Special: If you attack with a two-handed weapon, or with a one-handed weapon wielded in two hands, instead add twice the number subtracted from your attack rolls. You can’t add the bonus from Power Attack to the damage dealt with a light weapon (except with unarmed strikes or natural weapon attacks), even though the penalty on attack rolls still applies. (Normally, you treat a double weapon as a one-handed weapon and a light weapon. If you choose to use a double weapon like a two-handed weapon, attacking with only one end of it in a round, you treat it as a two-handed weapon.)
It's unclear from this whether "Normally, you treat a double weapon as a one-handed weapon and a light weapon" means "Normally,
for the purposes of Power Attack", or "Normally,
for all purposes".
But it can certainly be argued that it does, in fact, only apply to the Power Attack feat, which means that as written, a double weapon is treated as a one-handed and light weapon a/ for the purpose of attack penalties associated with two-weapon combat, and b/ for the purpose of Power Attack.
A double weapon used for a Disarm attempt, for example, would still gain a +4 bonus... since it is a two-handed weapon, and the -4 a light weapon receives in Disarms applies whether or not the wielder is engaged in two-weapon combat, and is thus not an attack penalty associated with two-weapon combat, and thus for purposes of Disarms, the double weapon is not treated as a combination of one-handed and light weapon.
-Hyp.