Abel wants to affect a shadow (MM 221-2) with the 3rd-level Clr spell spark of life [necro] (SpC 196). The shadow has the subtype incorporeal, and the spell spark of life is Range: Touch and doesn't deal damage.
While holding the charge from the spell spark of life Abel rolls high enough that his melee touch attack versus the shadow would be successful. What happens next?
- He hits! The shadow makes a Willpower saving throw. Because the spell spark of life is a spell--no matter its delivery method--and the spell doesn't deal damage, the shadow's incorporeal subtype's 50% immunity to corporeal damage sources doesn't affect the spell.
- He may hit. The DM checks the shadow's 50% immunity to corporeal damage sources amd, if Abel successfully overcomes that, then the shadow makes a Willpower saving throw. Because the attack is magic yet delivered by a corporeal source, the shadow's incorporeal subtype's 50% immunity to corporeal damage sources must be checked.
- He misses! The attack automatically misses. The attack is a nonmagical melee touch attack (although the spell's charge is magical). The shadow--immune to nonmagical attack forms--is totally safe from Abe's touch.
I'm almost certain #2 is wrong as neither touch attack nor spell deals damage, but it's included for completeness.
The Range entry for Touch says that
You must touch a creature or object to affect it. A touch spell that deals damage can score a critical hit just as a weapon can. A touch spell threatens a critical hit on a natural roll of 20 and deals double damage on a successful critical hit. (PH 175)
and the section Touch Spells in Combat explains further that
Many spells have a range of touch. To use these spells, you cast the spell and then touch the subject.... Touching an opponent with a touch spell is considered to be an armed attack and therefore does not provoke attacks of opportunity. (PH 140-1)
The subtype says that
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. (MM 310-1)
This question's answer and the Comments on its answer get really close to answering this question.