Disclaimer: The following post contains some ramblings and musings that may not be entirely focused or relevant.
While I generally agree with "guest's" assessment of the Lawful-Chaotic alignment axis, I disagree on one note. While *most* chaotic characters may lack the self-discipline or willpower, I don't think it's a "chaotic implies unfocused willy-nilly mentality." While cases like the Xaoisects (sp?) in PST certainly fall under that category, the way you're defining "Chaotic" doesn't necessitate a lack of discipline.
While it's generally agreed that a disciplined, law-abiding character is more lawful than an undisciplined, individualistic opportunist, being generally "lawful" essentially means the character displays significantly more lawful characteristics than chaotic ones, and vice versa. That being the case, it makes some sense that a generally chaotic character can possess the mental discipline necessary to follow the monk's path. However, since the traits of a monk favor law to chaos, you're likely to see few neutral monks and fewer chaotic ones. To simplify things and cover perhaps 95% of all monks, you could penalize unlawful ones.
Paladins, on the other hand, relying on the favor of a deity, are likely more restricted than monks (or barbarians, or whatever) in terms of alignment. Lawful Good gods are likely to look for a very strict set of traits in their chosen warriors, so paladins as a class will be made up of "the lawful of the lawful," so to speak.
Ultimately, I can definitely see the possibility for an argument in favor of a neutral (or *possibly* chaotic) monk. Neutral (*maybe* lawful) barbarians may also be justified. On the other hand, I think you'd be awfully hard-pressed (i.e. it's nigh-impossible) to come up with an explanation for a neutral (or worse, chaotic) paladin. That I can't come up with good examples simply means I'm not sufficiently creative.