Are incorporeal creatures immune to damage from conjuration (creation) spells like snowball, stone discus, or caustic eruption? None of these spells offer spell resistance, leading me to believe that they're mundane sources of damage brought into existence by spell energy. It would be like conjuring a piano and pushing it off a cliff onto a ghost. Magic was involved, but the attack isn't magical.
Rules for reference:
1. From the incorporeal rules: An incorporeal creature has no physical body. It can be harmed only by other incorporeal creatures, magic weapons or creatures that strike as magic weapons, and spells, spell-like abilities, or supernatural abilities. It is immune to all nonmagical attack forms.
2. From the conjuration (creation) magic rules: A creation spell manipulates matter to create an object or creature in the place the spellcaster designates. If the spell has a duration other than instantaneous, magic holds the creation together, and when the spell ends, the conjured creature or object vanishes without a trace. If the spell has an instantaneous duration, the created object or creature is merely assembled through magic. It lasts indefinitely and does not depend on magic for its existence.
A +1 sword doesn't offer spell resistance, but it still harms incorporeals...
Incorporeal creatures take half damage (50%) from magic weapons, spells, spell-like effects, and supernatural effects.
So it has nothing to do with spell resistance (since Su effects get the same treatment).