It looks like there are several active topics here. Arcady, your original post says that you have a problem with how your players are treating the caste social system in your game, which is to say they have a total disregard for nobility and the order of the game. I say players instead of characters because I'm confident your beef is with the behavior of the players, not the alignment or background upbringing of their character. I have a couple of questions if this is the case.
First, are any of these characters lawful in alignment? I don't want to get into a heated debate about what exactly "lawful" is, but I feel personally that Lawful PCs are supposed to respect the established order of things, if the society itself has strong structure. Any players that have lawfully aligned characters facing a truly lawful ruler should have a little talking to about alignment. At this point I should make the disclaimer that if the group has a paladin, and there's a tyrant on the throne, I would say the paladin or any L/G character should be working to overthrow that tyrant. As a counterpoint any L/G character should still show proper respect simply due to their lawful nature.
Secondly, were the characters raised away from this type of society? I know the answer is probably no, but if these characters are the type of wilderness people that literally live under a rock and don't interact with society, you seriously can't expect much.
Lastly, and this is where most of the conversation has been going, how powerful (game mechanically) are these characters? This is a trick question, because it really shouldn't matter. The key here is the players that are running these characters may feel their characters are so powerful they no longer need to give this respect. It may not have anything to do with the actual power of the players, but in my experience players running 1st-5th level characters don't irritate a lot of NPCs.
It is the players that need to be taught to have this kind of respect in their game, and that requires a healthy dose of FUD.
There are two ways to teach players: actively and passively. As the DM, you directly control every aspect of the game, except for the minds of the players. You can take them aside and tell them of your expectations of their behavior in this social system. Unfortunately, in my experience that tends to create resentful players who will make characters and behave just barely within your guidelines and frustrate you. Not all players will behave like this but in this situation I can forsee the strong possibility it will. A good example I can draw from was in my old monotheistic game, where my wish was pretty simple: everyone worships Mystra (the one goddess). I'd say about 50% of the players immediately made up their minds to not worship her and be apathetic about the whole situation. The situation had to be rectified by not allowing healing spells affect them from priestesses. Of course, inflict wounds worked great. This method tends to butt heads with the players, and makes the gaming experience less enjoyable overall. This is also why DM's that railroad their players into plot threads commonly end up with dissatisfied players. It also helps to be proactive and not reactive, as seen in prior posts.
Passive teaching works wonders on players whose "style" of role-playing differs from yours. As I said before a healthy dose of Fear, Uncertainty, and Doubt will control your players without them feeling like they're being manipulated. For example, my players also have this problem with authority. Granted, they're all Chaotic Good or C/N in alignment, but I agree that a certain amound of respect, even if it's insincere, should be given by the players to their respective lords. The few days before these characters traveled to a warlord's keep, word of their pilgramage took like wind to the villagers. Soon word got back to the party that they probably shouldn't go visiting Azukhar since he's a vampire. Is it true? Do the players really want to find out? To top things off, Azukhar arranged their visit to take place at night. Throw in a few lines from Bram Stoker's Dracula in their dinner conversation with Azukhar, and FUD was assured. Is Azukhar a vampire? And who's that robed half-orc standing next to his throne? This may not be your style of GMing, but it can be used to a lesser degree.
Subtle hints go a long way to observant players. As a noble would, show the power of their wealth/militarty strength in parades, or "horse maneuvers", where players would see a dozen mounted knights engaging in sport. Be less subtle; have a supplicant address the noble as you would have it done, and show that the supplicant recieves what he wanted, even if the noble is a jerk or even evil. If you haven't run a horror RPG lately, I strongly recommend it. It is a great challenge for a GM because you have to make the players afraid. It helps refine your control over the players sympathies and emotions. "The strongest human emotion is fear, and the strongest kind of fear is fear of the unknown."
Like B.F. Skinner, monitor the condition/response of your players and you will have control of their actions almost unconsciously.
As an afterthought, I hope nobody takes this post as an insult to their intelligence or DMing abilities. I'm not trying to be patronizing, just trying to help.
