Forgot to post this before, but I experienced with my last character that the cold damage from my Barbarian's arctic rage will still do damage in spite of elemental reduction. When I used a weapon with two forms of elemental damage, one would be absorbed while the other would not. At least two people told me they experienced this.
Damage Increase (//%22http://nwn.wikia.com/wiki/Damage_increase%22)
Physical damage (//%22http://nwn.wikia.com/wiki/Physical_damage%22) increase effects are converted to the wielded weapon's base damage type and are subject to damage reduction (//%22http://nwn.wikia.com/wiki/Damage_reduction%22), damage resistance (//%22http://nwn.wikia.com/wiki/Damage_resistance%22) and damage immunity (//%22http://nwn.wikia.com/wiki/Immunity%22) as usual. Other damage types ignore the damage resistance and immunity of creatures, but are individually subject to the hardness (//%22http://nwn.wikia.com/wiki/Hardness%22) of a placeable (//%22http://nwn.wikia.com/wiki/Placeable%22).
This is why it will work on creatures with Protection from Elements but not on a door with a hardness rating.