The incorporeal entry says:"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. Even when hit by spells or magic weapons, it takes only half damage from a corporeal source (except for channel energy)."
So, can a dragon, who has DR damage an incorporeal creature with its natural weapons?
The dragon entry only says, its natural weapons count as magical for the purpose of overcoming DR.
But, for example, a shadow doesn't have DR...
And how about the Dragon's Breath Weapon?