Is there any damage type that cannot be resisted?

Question

In other words, if each of your resistances provides 20% damage reduction from it's respective damage type, is it the same as having 20% damage reduction from armor?

Answer 1

There is no damage type that cannot be resisted, so yes, 20% reduction from resistances is equivalent to 20% reduction from armor.

And as fbueckert pointed out, damage reduction is taken sequentially, not additively.

Answer 2

As of patch 1.0.2, the attacks of Lacuni Huntresses found in act 2 ignore all armor and resistances. But flat damage reduction (ie, all melee damage reduced by 20%) works correctly against them.

This includes damage from mortar, plague, desecrated, arcane enchanted, frozen, reflect damage affixes if they are spawned with them.