Gaiden said:
The alternative is to treat the brilliiant energy property as transforming the blade and not just enhancing it such that now the blade is light INSTEAD of whatever it was before. If you do this you have to follow this interpretation through to its conclusion, however. If you are treating the blade as made out of light, it would technically count as energy in the same way that searing light, the lantern archon's ray attack, sunbeam, etc. would count as energy - meaning DR was irrelevant. As light it would count as an non-descriptor type of energy so that no energy resistance would apply. This interpretation makes a brilliant energy weapon perhaps more powerful than it was before - or for a +4 virtual enhancement is perhaps exactly as powerful as it should be considering the rarity of armor and shields on high CR monsters. YMMV
I am thinking really hard about this....
No, not really. I have a slightly different campaign world set up, so this isn't a problem. But here's support for both sides in the RAW:
SRD said:
Brilliant Energy: A brilliant energy weapon has its significant portion transformed into light, although this does not modify the item’s weight. It always gives off light as a torch (20-foot radius). A brilliant energy weapon ignores nonliving matter. Armor and shield bonuses to AC (including any enhancement bonuses to that armor) do not count against it because the weapon passes through armor. (Dexterity, deflection, dodge, natural armor, and other such bonuses still apply.) A brilliant energy weapon cannot harm constructs, deathless, undead, and objects. This property can not be applied to ranged weapons, but can be applied to ammunition.
Strong transmutation; CL 16th; Craft Magic Arms and Armor, gaseous form, continual flame; Price +4 bonus
It gives off light because it uses
continual flame as a part of its creation, which doesn't do any kind of energy damage. A brilliant energy weapon passes through armor because it uses
gaseous form in its construction.
SRD said:
Gaseous Form
Transmutation
...
The subject and all its gear become insubstantial, misty, and translucent. Its material armor (including natural armor) becomes worthless, though its size, Dexterity, deflection bonuses, and armor bonuses from force effects still apply. Creatures in gaseous form have damage reduction 10/magic. Spells, spell-like abilities, and supernatural abilities affect them normally.
But natural armor still applies. Why? Well, I don't see it in the SRD but I do recall reading that a creature's natural armor was both due to it's tough hide or shell and it's inherent density or underlying structure, so I would think at least some of the Natural Armor bonus is due to living material, and trying to split that hair would be a little silly, even for this message board.

On the flip side, and in answer to the Ex or Su for damage reduction question raised earlier:
MM Errata ?? said:
Damage Reduction is either extraordinary (Ex) or supernatural (Su). Use the following guidelines if it is not specified.
• DR X/slashing, piercing, or bludgeoning should be (Ex)
• DR X/adamantine should be (Ex)
• DR X/-- should be (Ex)
• DR X/silver or cold iron should be (Su)
• DR X/magic should be (Su)
• DR X/chaotic, lawful, holy, or unholy should be (Su)
So it looks like in the RAW, you can have an Adamantine Brilliant Energy weapon that loses it's properties, but silver and cold iron weapons should retain their qualities, sinde the damage reduction being bypassed is Supernatural. If it lost that quality, then a Holy Brilliant Energy Morningstar should lose it's ability to beat down creatures with the Evil descriptor. But there is one more thing to look at:
SRD said:
Gaseous Form
Transmutation
...
Continuously active items remain active, though in some cases their effects may be moot.
What could be moot about damage reduction? Well, if the material part of the blade is replaced in the majority by light, then the weapon is no longer mostly constructed of cold iron, and therefore no longer gains the benefits of being constructed of that material. Otherwise you could etch your name in the blade of your longsword in silver and whack away at the lycanthropes with more cash in your pocket.
LOL
Looks like the rules are taking turns contradicting each other.
In my campaign I add a little balance by confusing things further:
House rule said:
DR Type and Equivalent Enhancement Bonus
/magic +1
/crystal, /jade, or /obsidian (OA) +2
/silver or /cold iron +2
/adamantine or /wood +3
/bludgeoning, /piercing or /slashing +3
/lawful, /chaotic, /good or /evil +4
/epic +6
Thus, a +2 longsword or better inflicts normal damage against a werewolf, a +3 battle axe or better inflicts normal damage on a xorn, and a +4 mace or better inflicts normal damage against a titan.
These DR types stack to determine the minimum magical bonus of a weapon, so a balor, with damage reduction 15/cold iron and good, will have its damage reduction bypassed by epic weapons. A character fighting the Xixecal, with damage reduction 20/good and epic and adamantine, would need a +13 or better unaligned steel weapon to inflict normal damage.
That works for me and mine, since they mostly skip the enchantments and just go for the better bonus on the blade. Yes, it's a throwback to 3.0, but it works. Enjoy the predicament.