Does Incorporeal Subtype protect against the divine damage from Flame Strike?

Question

So my party encountered wraiths the other day for the first time and the Favoured Soul used Flame Strike, also for the first time. I was a bit stumped by the half "divine" damage as I've never seen it anywhere else. Is it positive damage? Has it been errata'd anywhere? I ruled to not allow the wraiths their 50% chance to ignore it as divine vs undead seems appropriate, but is there any RAW out there to support this decision, or is there another divine damage spell or effect I should be aware of when dealing with incorporeal creatures?

Relevant section of Incorporeal Subtype:

Even when hit by spells or magic weapons, it [an incorporeal creature] 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).

Relevant section of Flame Strike:

Half the damage [of the spell] is fire damage, but the other half results directly from divine power and is therefore not subject to being reduced by resistance to fire-based attacks, such as that granted by protection from energy (fire), fire shield (chill shield), and similar magic.

Answer 1

The rules as written don't support your decision…

Divine damage—like half the damage dealt by the 5th-level cleric spell flame strike [evoc] (Player's Handbook 231)—is just damage with an unusual descriptor. It has no additional or special effect against anything unless a game element specifies that it's affected differently by divine damage. There's no errata because there's no need for errata—it just is.

Presumably, the favored soul was himself corporeal when he caught within the area of that flame strike spell that normally incorporeal wraith (Monster Manual 257—8). With this in mind, the Rules Compendium on Incorporeality on Harming says

Even when hit by spells (including touch spells) or magic weapons, an incorporeal creature 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. (64)

As the divine damage from the spell flame strike is none of these things, the game would have the DM flip a coin and if the DM wins, the entirety of the favored soul's flame strike spell doesn't affect the wraith. If the DM loses the flip, the entirety of the favored soul's flame strike spell affects the wraith normally—it checks spell resistance (if any), makes a Reflex saving throw, and so on.

…But you've set a precedent so walk it back or accept it

It's extremely difficult for unprepared parties to deal with incorporeal creatures. The first appearance of an incorporeal creature in a campaign can lead to at least one—if not several—characters' deaths. If that's what happened here, it's totally okay to have the favored soul's flame strike spell deal half its damage to the incorporeal wraith as a gift from the favored soul's god or whatever. Maybe you'll want to rule that way, the god visiting the favored soul in his dreams, saying, "Next time, be prepared!" or maybe you'll just want to keep the divine damage of the flame strike spell naturally affecting incorporeal creatures for the the campaign's duration. While this latter option provides a very small power-up to divine casters—druids, for example, get the spell flame strike as a 4th-level spell—, this likely won't have any long-term campaign consequences until the shadowpocalypse.

Answer 2

The only special quality the half-damage resulting "directly from divine power" is that it is not subject to any fire-damage weaknesses. This dm would rule that it is basically all fire damage, but the "divine power" behind half of it prevents resistance. That way, things with vulnerability are still affected since the spell doesn't explicitly say fire vulnerabilities are likewise ignored.

Think "God made this thing so hot that it burns everything." Given the information above, it's definitely not positive energy or anything like that.

Any resistances that are not explicitly to fire damage are not affected by the spell.

To answer the question: yes, incorporeal protects against flame strike just as it protects against other spells because the protection is not specifically vs fire.