From the 3.5E SRD:
Incorporeal Subtype: 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. Even when hit by spells or magic weapons, it has a 50% chance to ignore any damage from a corporeal source (except for positive energy, negative energy, force effects such as magic missile, or attacks made with ghost touch weapons). Although it is not a magical attack, holy water can affect incorporeal undead, but a hit with holy water has a 50% chance of not affecting an incorporeal creature.
Cure Light Wounds
When laying your hand upon a living creature, you channel positive energy that cures 1d8 points of damage +1 point per caster level (maximum +5).
Since undead are powered by negative energy, this spell deals damage to them instead of curing their wounds. An undead creature can apply spell resistance, and can attempt a Will save to take half damage.
Does this mean that a touch attack on an incorporeal undead by a Cleric casting a cure spell (or a Paladin laying on hands) does not have a miss chance, since the attack is using positive energy?Paladin class description
Alternatively, a paladin can use any or all of this healing power to deal damage to undead creatures. Using lay on hands in this way requires a successful melee touch attack and doesn't provoke an attack of opportunity. The paladin decides how many of her daily allotment of points to use as damage after successfully touching an undead creature.