I find it interesting that people who play Chaotic Nuetral do not believe that they are by choosing that alignment saying, in effect, that they intend to protray a character who has a radical and extreme moral and ethical system. Because, they are.
Consider the following. If a player states that the philosophy of his character is NG, NE, or LN, every DM immediately understands that the character is intended to be uncompromisingly good, evil, or lawful, respectively, and understands that when the character acts otherwise the player first needs to be warned, then perhaps counciled, and if no compromise can be reached the characters alignment needs to shift appropriately. Alignment change is intended to be a penalty for poor roleplay, especially when it occurs for no dramatically important reason (redemption, transformation, transgression, etc.).
If a character is supposed to be CN, then he is going to have a very hard time getting along with a group and not suffering an alignment change. If I had a player in my campaign, and the player chose Chaotic Neutral for no better reason than he thought it would allow him to avoid consistant and well reasoned role play, then I would be quick to point out to him that the following actions were violations of his stated alignment whenever they occurred:
1) Agreeing to allow someone else to lead, or consistantly following someone elses suggestions (even if they were reasonable or that other person clearly was the best person to lead). There is no such thing as a chaotic neutral follower. Every CN believes that it is his right to rule the world.
2) Not acting in his own best immediate self-interest. In other words, not attempting to cheat, steal, lie, convive, or gather more than his share of the glory, wealth, and prestige of the group. Would you allow the Paladin to lie, cheat, and steal without attoning? Then why do you allow a CN to get away with not doing it?
3) Not acting contrarily, randomly, and capraciously. That is to say, not occasionally doing something merely because someone forbid him to do it implicitly or explicitly. You wouldn't allow a lawful character to be disobiedient to those in authority, why would you allow a chaotic character to be obedient and subservient?
4) Not acting illogically, spontaneously, and with very little reflection. That is to say, not occasionally doing something that had very little or no chance of success (and was probably dangerous) merely to make things interesting or see what would happen. (Or merely because he believed that life was so unpredicatable that no well thought out plan had any more chance of success than a random and seemingly unreasonable one.) You wouldn't allow a lawful character to be always brash and unthinking would you?
5) Being loyal. Not betraying his friends when there was a clear benefit to doing so. You wouldn't allow a lawful neutral to betray someone without taking some alignment penalty would you?
6) Working for or taking actions that assist a lawful aligned group, person or agency even when such actions are beneficial to himself. If working for a lawful agency, the character must ultimately betray the agency or violate alignment. You wouldn't allow a good character to work for diabolists because they allowed him to further some good cause?
Although I would accept some deviation on one point or another because people are different and can hold thier beliefs to greater or lesser degrees, consistant deviation on all or many points is a sign that something is wrong.
A chaotic neutral character must set his own interests first, or he is not chaotic neutral. Chaotic neutrals do not generally form long friendships. If your player argues that his character forms a lasting friendship with someone else because it is in his best interest to do so, point out that the principal he has just stated is THE ESSENCE OF THE LAWFUL PHILOSOPHY. It is the lawful philosophy that says the most success occurs to the most people when everyone works together. To state that your character believes that his best interest is served by being loyal to someone else is to state that your character is not chaotic neutral.
Would you accept the arguement from a nuetral good character that the ends justify the means? That he is only committing this evil act in order to further a good? I wouldn't. To claim that good had to be evil in order to accomplish good is to deny the essence of belief in goodness. The same principal holds true for True Chaotics and any other alignment group.
(As an aside, I have long argued that most people don't have a consistant set of ethical principals, and that one of the reason you see alot of people arguing that alignment is bogus is that in fact they, the players, are neutrals and don't believe in real life that ethics and morals really exist.)
A well played party of CN's should be bickering, lawless, back stabbing anarchs, each always trying to put themselves to the front, and probably should not stay cohesive for long as each is uncomfortable except as a lone wolf. It could be fun, but it's probably not what the players of CN's you describe want, which is perfect freedom of action without penalty. CN want perfect freedom of action (True), but they accept nothing less.
Most characters have nuetral alignments in my experience. They don't really strongly hold to any particular principal and will conform thier actions to make the most of the surrounding situation - lying and murdering when necessary, and acting selflessly and heroicly when necessary. Whether this is the result of the underlying philosophy of the player coming through, or the result of treating action in an RPG like moves in a game of chess (which need no justification other than logic and the pursuit of success), I don't know. However I do know, that if you want to include philosophical elements in your campaign, anytime a player trends excessively in a moral direction other than thier stated one, you as a DM need to step in and provide some guidance either as the DM or as the player's concience, deity, or whatever.
(Another aside - as a person who believes his own philosophy is best described in game terms as NG, I tend to feel that the reason that we have a hard time distinguishing between evil actions and chaotic actions is that 'chaos' and 'law' are abitrary, secondary, distinctions. A similar falacy can be seen when you ask the average person which alignment is 'the most good'. Most players respond 'LG'. In fact, the answer is NG, or 'Pure Good'. LG's, like paladins, are tainted by lawfulness and more prone to nongood actions (in the service of law) than NG's. Most people falsely associate goodness with orderliness, obedience, and so forth, when those are just arbitary and secondary distinctions. Running any of the moral neutral alignments is therefore very tricky, since IMO such categories don't really exist. An ammoral person is IMO no different than an immoral person. Just because he doesn't believe in 'good' and 'evil' doesn't mean that such concepts don't exist. As a long time player of a Chaotic Neutral, I found it hard to distinguish my characters actions from Evil except by secretly in my head giving the character a set of rules to follow. A secret code of honor, as it where. For instance, balancing his freedom to steal (a chaotic evil action), I decided that he felt that everyone had the right to certain basic freedoms (a chaotic good principal), and would not seek vengence on someone for doing something that he would have done himself (for instance stealing some of his own minor possessions), and would not enslave someone else (no charm person, liberate slaves at a whim, etc.). However, at its heart this orderly approach is a betrayal of chaotic. Honor is generally deemed a lawful principal. CN's don't give themselves guiding principals. I tried to get around this by telling myself that he (unlike his player) was acting out of emotion and not logic, but it was never perfectly satisfying.)
