Another (somewhat long) thought on the matter:
Relative to magic, the primary strength of psionics is the gathering and relaying of information. The impact this has on a society depends on how widely-available this ability is. For the purpose of this little thought-experiment, we will first assume that the only psionic people will be those with class levels in the PC psionic classes. We will then assume that there is a psionic equivalent of the adept (the psychic).
We will probably see two distinct courses of development, directly related to the natures (i.e. alignments) of the earliest psionic characters. Evil psionic characters would likely abuse their power to attract followers and to make claims to godhood to abuse the gullibility of the masses, their own egos being spurred by their percieved power over others. Good characters, on the other hand, would likely experience even more empathy for others, and they would likely attempt to organize and better society as a whole in the ways they saw best. This early phase would likely mirror the "ancient mage-king" phenomenon, where someone who wields powerful magic would come to power and shape society magically (or in this case, psionically).
So we have now arrived at a society goverened (in one way or another) by a psionic character (or psionic characters). It is here that the distinction as to how much of the society can "tap" the resource of psionic powers comes to matter. If it remains the tool of a few, then psionics runs the risk of becoming the deciding factor for political or social character, as has already been predicted. Assuming rarity assumes a psionic elite would arise.
Assuming randomness goes against this, since the society would have difficulty "controlling" who gains psionic abilities. Naturally, a society could take measures against this (the mutant society ruled by Apocalypse in the distant future of the X-Men universe answers this by having children tested at birth for an X-factor mutation, and even Xavier's Cerebro machine could be said to have this function)... in this situation, a psionic power or device designed to seek out or identify psionic ability from a young age could help to overcome randomness. Regardless of this, truly random psionic potential would lead to societies being governed by oligarchies, since there would be no garuntee that one leader's offspring would inherit the necessary psionic power to maintain his position. These societies would probably be similar overall to how the Greek city-states developed.
Having psionic ability being heritable could lead to bloodlines of powerful psions, which would in turn encourage monarchies to develop. This would lead to a society more akin to what we would consider a classic fantasy world, with noble families, royalty, etc.
Regardless of its heritability, "psionics is rare" would lead to the consolidation of power over information in a few small enclaves of power, and a significant erosion of individual freedoms. The proliteriate would enjoy less power than in equivalent societies.
The opposite scenario would be one where psionics are more common (at least as common as magic as magic in 3.X D&D, with an NPC psychic class). In this situation, psionics would democratize information. It would be much more difficult for a society to control the flow of information if a sizeable minority of the population could manifest psionic abilities. Further, it would also include regulations on how these powers can be used, since a central power would no longer have a monopoly over psionic talent: since they would be just as harmed by abuse of this power, they would attempt to curb its potency through legislation. In this situation (the one my own setting assumes), society would differ primarily from traditional magical medievial societies in that information would flow more freely and more quickly. Information would likely flow about as quickly as it did after the advent of the telegraph.
Clairsentience and telepathy are the two big factors here, since both of them are likely more potent means of divining information than traditional magical divinations (many of which are subject to the whims of fickle creatures of the outer planes). It would be very easy for NPC psychics to link to one another in a small communication network (expanded knowledge: mindlink and craft universal item can allow a psychic to very easily and cheaply create an item to maintain instant communication between two people). Such a communication method would probably be maintained among small group of psionic characters, and made open to the public (likely at a fee as a sort of messenger system).
The other issue here is the ability for psionic characters to know precisely what others are thinking. With a significant psionic population, as suggested above, it would either be illegal or at least taboo for psionic characters to go picking through the minds of others. However, some things probably would be acceptable: reading or altering emotions and the like.
...
That turned out to be a bit more rambling than I would've liked. Ah well.
