For citation, consider the DMG sample NPCs and you'll see their spellcasting has been modified by the ability-boost magic items they own. (Looking at 3.0 myself.)
It is pretty much a general rule: ability scores for spellcasting, feat pre-reqs, even spells for item creation, you can use items to provide them. Of course, if you lose the item, you lose any abilities it was allowing.
I can't give you a specific citation for that in the core books, but I can point out that Int modifying items don't provide extra skill points, because it specifically says so in the item description (and the skill point bit). That strongly implies to me that where it doesn't have such a restriction (pretty much everywhere else), there is no difference between getting 14 due to an item or not (as long as nothing happens to the item).