Incorporeal creatures are only damaged by magical effects (50% of the time), force damage, other incorporeal things, and things specifically designed to interact with incorporeal creatures (e.g. Ghost Touch weapons).
The Dreamheart is, well, the heart of the Plane of Dreams, and as far as I can tell is mostly covered by the Dreamheart Tempest, an ambient storm of energy that does 5 damage of each elemental type (fire, acid, cold, lightning, and sonic) every turn to a character that hasn’t found shelter.
How do these two things interact?
Take the Dream Vestige (Libris Mortis), an incorporeal undead that can enter the Plane of Dreams at will. Is it immune to the Tempest because it’s not called out as a magical effect, just an ambient part of the landscape? Or is elemental damage magical by default, and the Vestige risks being shredded every time it ventures too deep into the plane?