Disintegrate affects force effects like wall of force. OK. Are incorporeal beings force effects ? I don't think so.
Here's the complete definition of incorporeality:
INCORPOREALITY
Incorporeal creatures can only be harmed by other incorporeal creatures, by +1 or better weapons, or by spells, spell-like effects, or supernatural effects. They are immune to all nonmagical attack forms. They are not burned by normal fires, affected by natural cold, or harmed by mundane acids.
Even when struck by magic or magic weapons, an incorporeal creature has a 50% chance to ignore any damage from a corporeal source—except for a force effect
Incorporeal creatures move in any direction (including up or down) at will. They do not need to walk on the ground.
Incorporeal creatures can pass through solid objects at will, although they cannot see when their eyes are within solid matter.
Incorporeal creatures are inaudible unless they decide to make noise.
The physical attacks of incorporeal creatures ignore material armor, even magic armor, unless it is made of force or has the ghost touch ability.
Incorporeal creatures pass through and operate in water as easily as they do in air.
Incorporeal creatures cannot fall or suffer falling damage.
Corporeal creatures cannot trip or grapple incorporeal creatures.
Incorporeal creatures have no weight and do not set off traps that are triggered by weight.
Incorporeal creatures do not leave footprints, have no scent, and make no noise unless they manifest, and even then they only make noise intentionally.
And of disintegrate:
Disintegrate
Transmutation
Level: Destruction 7, Sor/Wiz 6
Components: V, S, M/DF
Casting Time: 1 action
Range: Medium (100 ft. + 10 ft./level)
Effect: Ray
Duration: Instantaneous
Saving Throw: Fortitude partial
Spell Resistance: Yes
A thin, green ray springs from the character's pointing finger, causing the creature or object it strikes to glow and vanish, leaving behind only a trace of fine dust. The character must make a successful ranged touch attack to hit. Up to a 10-foot cube of nonliving matter is affected, so the spell disintegrates only part of any very large object or structure targeted. The ray affects even magical matter or energy of a magical nature but not a globe of invulnerability or an antimagic field. A creature or object that makes a successful Fortitude save is only partially affected. It takes 5d6 points of damage instead of disintegrating. Only the first creature or object struck can be affected (that is, the ray affects only one target per casting).
No, it all boils down to whether an incorporeal being is made of "magical matter" or no, or of "energy of a magical nature" or no.
Incorporeal undead could arguably said to be made of negative energy. So they are adressed by the "energy of a magical nature". Now, on incorporeal creatures that are not undead, like, say, a spectral panther in its spectral form. Is it also made of energy of a magical nature ? What energy ? Positive ? Something else and undefined ?
Also, I would prefer to save "magical matter" for ethereal beings. Ethereal beings who confuse things up because of ghosts and manifestation.
While on the Ethereal Plane, a creature is called ethereal.
Ethereal creatures are invisible, inaudible, insubstantial, and scentless to creatures on the Material Plane (the normal world). Even most magical attacks have no effect on them. See invisibility and true seeing reveal ethereal creatures.
An ethereal creature can see and hear into the Material Plane in a 60-foot radius, though material objects still block sight and sound. (An ethereal creature can’t see through a material wall, for instance.) Things on the Material Plane, however, look gray, indistinct, and ghostly. An ethereal creature can’t affect the Material Plane, not even magically. An ethereal creature, however, interacts with other ethereal creatures and objects the way material creatures interact with material creatures and objects.
Ethereal creatures move in any direction (including up or down) at will. They do not need to walk on the ground, and material objects don’t block them (though they can’t see while their eyes are within solid material).
Force effects are a special exception. A force effect extends onto the Ethereal Plane. Gaze effects and abjurations also extend from the Material Plane to the Ethereal Plane. None of these effects extend from the Ethereal Plane to the Material Plane.
Ghosts have a power called manifestation that allows them to appear on the Material Plane as incorporeal creatures. Still, they are on the Ethereal Plane, and another ethereal creature can interact normally with a manifesting ghost.
Ethereal creatures pass through and operate in water as easily as air.
Ethereal creatures do not fall or suffer falling damage.
