After repeated tests, I have concluded that invisibility potions are no longer effective. I first noticed this issue some time during one of the 1.7 releases, and a search through forums has revealed it to have been a problem since at least version 1.6.4
Other players report that AI still responded to them as if visible, including a range of all common hostile mobs except spiders, but in my own experience even spiders responded in agro upon coming into usual proximity with me.
Cannot confirm. Did you have an item in your hand when this occured?