I saw that. However, that's not what I had in mind. Arcana in the skill challenge represents having a great deal of specific knowledge of the slaad. I think that there is some advantage ought to be gained to having a great deal of knowledge about philosophy, religion, rhetoric, etc. generally. You might not know anything about slaad, but you might recognize something about how this creature thinks from its responses and be able to respond with things you learned about Absurdism, Discordianism, Existentialism, a Zen koan (or its equivalent in your world), or a Sufi mystical saying (or its equivalent in your world) and thereby demonstrate first that you are perhaps as enlightened as the creature (or as it believes itself to be) and second that your are capable of responding to its quite perceptive comments (as the slaad believes them to be) like something intelligent (as far as the creature is concerned).
I agree, and would bring in Religion as an additional skill to use. Now, I have also house-ruled that a few additional skills exist (even if just in spirit, rather than mechanically).
In my old 3.x game, I gave each PC a "hobby" skill that could be a Knowledge, Perform, Craft or Profession skill that they automatically gain as a class skill and are given free Skill Focus in that skill. Caveat -- the skill must be something that is actually like a hobby and can't be something that you would normally take as a classed PC. For example, a wizard can't take Knowledge:Arcana, but he can take Perform: Lute or Craft: Woodworking. Yes, it was a subjective DM fiat thing, but the players got the idea.
I think the slaad cease to exist in every meaningful without their animating principle. If the animating principle is abandoned, they are just silly monsters. And if the animating principle remains, then you've still got alignments you are just refusing to acknowledge it or adopting a new terminology for labeling it. It's as meaningless as saying that modrons must be good simply because they are lawful. If you say that, then the modrons cease to be modrons in any meaningful way.
The only 4e alignment that remotely makes since for the slaad is Unaligned, if only because the Slaad don't really declare allegiance to anything and none of the 5 options are descriptive. But Unaligned is silly on the grounds that the slaad don't have to declare allegiance to chaos the way a mortal might; they simply are chaos. They are the thing itself. They are chaos without evil, and if they aren't then you are saying that they can't be themselves. If you want to deny that they are what they are, the question remains, what are they? And if they are something, why can't mortals declare their allegiance to it?
Whew.... yes I agree that if there was one thing that the alignment system of previous editions did right was to qualify certain creature attitudes. The Slaad are a very specific example where Chaotic Neutral really played a role beyond the "detect/smite" type identify/defeat spells and abilities.
My head, like Andor's also exploded with your explanation.... Consider that a good thing. Very chaotic neutral.
But.... I think that you can have a Slaad Unaligned and still exhibit the kinds of chaos-personified that you want. Unfortunately, in 4e it is not codified and would require a skilled DM to pull off correctly.
You, for what it's worth, have a lot more practical experience with Slaad than most, and can most accurately quantify and qualify the alignment and the personality requirements to properly NPC the creature.
But (also a houserule) I have adopted an uncodified Law-Chaos axis that I like to use in my games. I like the Elric/Corum/Hawkwind philosophy of Law/Chaos that Moorcock described. the players like the idea too.
So I would make the Slaad an Unaligned creature with a super-sexy Chaos suit on.
