Empirate is generally correct (although it is debatable), but you are only half right.
Following your example, lets say you use a magic item to raise strength to apply for Power Attack, then take a PrC that requires PA, then lose the magic item (orr get Enfeebled, poisoned, whatever). When your strength is below 13, you have lost the ability to use Power Attack, but you have not actually lost the feat. You have still spent the slot on the feat, and still possess it for the purposes of qualifying for the PrC. You simply cannot make use of the feat (or any other feat that requires 13+ strength).
This is an important design consideration, and it is very clearly delineated in the way prerequisites are written. Note that all feats that build on other feats have the same base ability scores (ex: Great Cleave specifically lists Str 13+ as a prereq, despite the fact it also requires Power Attack), but PrCs prereqs are never written with an ability score (ex: Shadowdancer requires the Dodge feat, but not Dex 13+). This is an important distinction, and understanding it is critical to properly designing new feats/classes/whatever for v3.5. The reason for this distinction is to stop the exact scenario you mentioned where Ray of Enfeeblement (or poison, etc) becomes a save-or-die spell.