CRGreathouse said:
Which Outsiders would this apply to? Just N/NN/TN? NG/N/LN/CN/NE? Do you see what I'm getting at?
I'll step aside a bit from the other answers, and say that what it would apply to depends on the purpose for referencing the type.
If you're talking about magic (Protection from Evil spells, Bane weapons, and the like), in standard D&D magic seems able to interact with a creature's moral and ethical karmic balances. The long-term sum of actions leaves a "residue" upon a creature that interacts with some magics.
In this way, creatures that are "neutral" in the sense that they have no committment either way to Law or Chaos have no noticble amount of this "residue". One might posit that a outsider that is committed to specifically maintianing balance between Law and Chaos might have yet another type of residue. In this case, if it exists at all, the Outsider(Neutral) type would choose out only those Neutral outsiders who are specifically acting to create balance, as opposed to those who don't give a hoot.
However, if you're talking about the Ranger's Favored Enemies bonus, I think you're talking about a different kettle of fish. The Favored Enemy bonus is based upon study of behavior and physiology. The ranger picks these by type, implying one of two things: 1)All creatures of a given type (or sub-type) have similarities of behavior and physiology, or 2)the character is assumed to have studied the details of an unrelated group of critters for some reason.
If (2) were true, then there's no real reason to stick to DMG types, except for convenience. The player could just as easily choose some other theme, and say, "I have studies creatures X, Y, and Z, and wish to apply my FE bonuses to them."
If (1) is true, then the taxonomy of outsiders is... Well, a bit odd.

I mean, the LE ones share traits with LG, and also with CE? But it's magic, so a bit of weirdness is okay.
But if all lawful outsiders have similarities, and all chaotic ones have similarities, it would stand to reason that ones Neutral with respect to those two would also have similarities. I'd then suppose you'd have two different classifications Outsider (ethically neutral) for Law/Chaos, and Outsider (morally neutral) for Good/Evil.