From SRD 3.5e
POWER ATTACK [GENERAL]
Prerequisite: Str 13.
Benefit: On your action, before making attack rolls for a round, you may choose to subtract a number from all melee attack rolls and add the same number to all melee damage rolls. This number may not exceed your base attack bonus. The penalty on attacks and bonus on damage apply until your next turn.
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.)
A fighter may select Power Attack as one of his fighter bonus feats.
From SRD 3.5e (again)
Rapier: You can use the Weapon Finesse feat to apply your Dexterity modifier instead of your Strength modifier to attack rolls with a rapier sized for you, even though it isn’t a light weapon for you. You can’t wield a rapier in two hands in order to apply 1-1/2 times your Strength bonus to damage.
所以：
1.「Power Attack」告訴我們除了天生武器和徒手打擊外的輕武器都不能PA
2.「Rapier」的說明告訴我們即使Rapier不是輕武器，但可以配合Weapon Finesse使用 (說明這是個特例)
因此：
從規則來說，Rapier (單手武器!!) 是可以PA的，只是你不能雙手握來獲得 2倍的PA加值 (還有 1.5倍的力量加值)
我想起了我的雙細劍PA遊俠 (因為堆到沒雙武減值，所以比起有雙武減值的人有多的AB可以開PA，不過效率另計...)
