As far as I know, only a player character's non-temporary ability scores are considered for taking feats, taking prestige classes, etc. Meanwhile, any ability bonus that comes from spells, or magic items that are in effect only while worn, is not counted (except tomes which increase abilities permanently).
So a Fighter with low Int who wants to qualify for Combat Expertise can't use spells or items that gives an Int bonus to raise it high enough to qualify. Is that right?
Say our fighter has Str 12, so he can not take the Power Attack feat. Suppose he asks for a spell from the wizard to make his Str 16, or to borrow a ring which provides +4 Str from another character. (Or any other method that will provide him a non-permanent bonus). His Str is now 16. Can he take Power Attack with that non-permanent bonus now? Because after the spell duration ends or he takes off the ring, his Str score will be to low again to qualify for Power Attack.