I am going by the definition of the incorporeal subtype from Monster Manual Ⅴ (page 213) and have a hard time coming to a definitive answer. I believe that incorporeal creatures should be able to execute touch spells, but how it describes interactions between incorporeal beings and physical things is a little ambiguous:
[An incorporeal creature] cannot take any physical action that moves or manipulates an opponent or that foe’s equipment, nor is it subject to such actions.
Simply: it cannot do onto others what others cannot do onto it. This is confirmed by Libris Mortis (page 141) where it talks about how the 50% miss chance also applies to corporeal creatures when it is the incorporeal making an attack:
Incorporeal creatures must deal with the same sorts of limitations on their ability to deal damage to corporeal targets as corporeal attacks do when fighting incorporeal foes.
But – back to MM5 – touch spells specifically work on an incorporeal creature:
Even when hit by spells (including touch spells) or magic weapons, […]
Is there any specific reason to believe that an incorporeal creature cannot execute touch spells back? Is touch defined anywhere as having to be physical?
I am asking for RAW answers here, which is why I ask for definitions and not interpretations.
Note that I am not talking about Ghosts. Ghost come into the Material Plane (while simultaniously staying on the Ethereal Plane) through the supernatural ability Manifestation. It is this ability that specifically bans using touch spells agains “nonethereal targets”.