The feat itself doesn't care how its prerequisites are met when the feat's taken. Employing spells and magic items to meet a feat's prerequisite means that when the spell's duration expires or if the magic item's lost the creature won't be able to use the feat anymore, but if that's a risk worth taking then it's legit to meet a feat's prerequisites using spells, magic items, or whatever. Also, a feat a creature took for which the creature's subsequently lost the prerequisites can still be used to meet the prerequisites of other feats, for example.
However, the difficulty comes in taking a feat in such a fashion in the first place. Usually feats are gained upon advancing a level, and
A character advances in level as soon as he earns enough experience points to do so—typically, this occurs at the end of a game session, when your GM hands out that session’s experience point awards.
This means to meet the prerequisites of a feat like Power Attack by using a spell like bull's strength the creature must have the spell bull's strength in effect when he gains those XP at the session's end! This isn't particularly shady ("Yay! I took a feat I can only use sometimes!"), but it is weird and the GM may still just say No, no matter how rules-legal the option might be.