The way the rules are written, they can take the Feat but only get the benefit while they still meet the requirements. But, this is just something I don't like about 3E. The whole bit about getting extra skill points for wearing a Headband of Intellect is the perfect example of the problem.
Let's say I decide to start a campaign at 10th level. Or, more likely, I have a campaign in progress and a new player joins. He makes a Human Bard with an INT of 14. Okay, that gives him 91 skill points, right?
"Oh, but I had a Headband of Intellect +2 that I bought back at 5th level, so I should get another 5 skill points, but I stopped wearing it this level."
Since I had no control over when he got that item (since it never happened in-game), can I say no? If I say no, he's less powerful than the existing characters, who HAVE been wearing those headbands. If I say yes, how do I balance it?
Same goes for NPCs I create. How many skill points do they get? In a low-magic campaign it's not bad, but if magic can give you permanent skill points?
And that's the problem I have. IMO, a character's abilities should be able to be calculated as-is with their current equipment, NOT dependent on what things happened in the campaign previously. Only things entirely under the character's control (when he picked spell X, when he increased stat Y) should be adjustable like that.
****HOUSE RULE WARNING!****
So, what we did IMC was separate attribute effects into "Innate" and "Temporary".
The Innate category includes your starting stat, the inherent bonus from a Wish spell, the bonuses/penalties for old age. Not much else. The Temporary category is everything else: spells, magical items, poisons, psionic attacks, rage, fatigue, etc. Since this includes both positives and negatives, this isn't a nerf.
Now, apply these rules:
Skill points use Innate INT. Hit points use Temporary CON (since they're retroactive this isn't a problem). Spell DCs use Temporary INT/WIS/CON, while Bonus Spells per Day use Innate INT/WIS/CON. Saves use Temporary CON/DEX/WIS. Feat prerequisites would use Innate stats (i.e., you need STR 13 to take Power Attack, but once you get it you can ALWAYS use it, even if your STR dropped below 13).
Quick rules of thumb:
> Sources: If you are hit by a Heal spell, stripped naked, and dumped in an Anti-Magic Field, would you still get the bonus? If so, it's Innate. If it goes away by anything less than a Wish spell, it's probably Temporary.
> Abilities: If it's something that doesn't make sense to go negative (number of uses per day of an ability, number of skill points gained, or number of bonus spells) it should use Innate. After all, if I cast an INT-boosting spell to memorize bonus spells, and then someone dispels the INT boost, do I lose spells? But, if it's something that can freely go negative (attack bonus, saving throws, save DC bonus, etc) use Temporary.