Creatures which are immune to poison damage are not immune to the non-damaging effects of powers with the poison keyword. However, they are immune to all effects of poisons described as items (for example, alchemical substances).
When poison is applied to a weapon or ammunition, a creature which is immune to poison damage is immune to all effects conferred by that poison. Effects which are not conferred by the poison (such as base weapon damage and non-damaging hit/effect properties of a power used to deliver the poison) are not affected by the immunity.
These changes are designed to make poison a more viable damage type, since full immunity to damage is normally very difficult to acquire.