mxyzplk's answer is mostly correct. It can apply to all, none, or just the attacks following turning Power Attack on.
To add to his answer, you can turn Power Attack on between iterative attacks. The language "You must choose to use this feat before making an attack roll" is worded so that you can't look at the die result and then determine if you want to use power attack.
You can take your first attack at normal BAB, then choose to turn power attack on for the following. There are some situations where this is useful. If you can only hit your first attack on a natural 18+, then you probably don't want to power attack. But that means your following attacks are only going to hit on a natural 20 so there's no reason to not turn on Power Attack at that point.
If "You must choose to use this feat before making an attack roll" meant only the first attack of the round it would have to say so, but it doesn't. A very literal (and very wrong) reading of this would also mean that a character could only Power Attack on his first attack ever (obviously that's wrong..).