IIRC, the rules are silent on this issue. I'm also not sure what the PFS ruling would be.
At my table, I allow this, with one major caveat. If you ever lose the item or otherwise are no longer under its effects, you lose the ability to use the feat.
So, say you have an 11 Int, and get a headband of vast intelligence +2. Assuming you wear it all the time, next level, you can take Combat Expertise as a feat. However, if you ever lose the item, or an enemy hits it with a targeted dispel magic, your Int drops back to 11, and you no longer qualify for the feat. That means you can't use it. That is also true for any feats that require it as a prerequisite. So, if you also had Improved Feint, you couldn't use that either.
Sean Reynolds said yes. Specifically, #2 in his post.
The rules do state that after 24 hours, the ability increase does increase the relevant ability.
Permanent Bonuses: Ability bonuses with a duration greater than 1 day actually increase the relevant ability score after 24 hours. Modify all skills and statistics related to that ability. This might cause you to gain skill points, hit points, and other bonuses. These bonuses should be noted separately in case they are removed.
So that means that with a Dexterity of 11, wearing a +2 Dex item for 24 hours will actually give you a Dexterity score of 13 for all intents and purposes.
Just don't lose/remove the item, or you lose the ability to use the feat.
