Basic, unenchanted armour, yes, splash potions ignore it completely.
And I can't speak for Projectile and Fire protection enchants as I do not use them much.
But I know that the Protection Enchant, when at Level 4 and if applied to a full set of Diamond Armour, negates most if not all forms of damage to a point where no damage taken is visible. ( I read somewhere on a MC Forum that it reduces 98% of all damage ) This not only effects Swords and Bows but also Splash potions too, it reduces the damage of Harming, Poison, etc.
I know that I was once poisoned wearing my set and I received no (visible) damage to my health at all during the time I was poisoned. Harming 2 Potions also cause very little damage when using that enchant. Slowness however works as normal and can't be stopped.