I doubt you'll find a rule stating you can't walk on water either. Unless stated explicitly in the rules, I presume the game world functions as ours. Sneak attack has a lot of flavor text and a reasonable explanation. It's clear how it interacts with basic D&D creatures. There isn't much else in there - certainly nothing about gullets, which clearly aren't your everyday creatures.
Granted, I can see your argument that the gullet is still part of the same creature (notably meta-information inferred from the real world), and that there are vital parts probably within reach from within the gullet (again, based on out-of-game reasoning). Unfortunately, it's possible to cut your way out of a D&D gullet without serious consequences for the creature. And then... muscular action (!) closes the hole. Clearly, we're in the realm of fantasy here, which is what D&D is all about.
I don't then feel comfortable inferring much from common sense. In terms of internal consistency, the existance of monsters who's survival strategy includes consuming creatures such that they're exposing themselves to harm just doesn't make sense. Further, there are a number of weak points in the sneak attack argument, using the plain old sneak attack description.
A sneak attack requires that the rogue can discern the anatomy. A rogue in a gullet cannot identify anatomy in the normal fashion, for he can perceive only a small fraction of the creature, if he can perceive anything at all. He most likely can't perceive anything at all, since it's dark, and full of digestive juices. The rogue fails at this first sneak attack prerequisite in three independent ways, therefore; he can't perceive clearly since he's restricted to perceiving the inside of the monster and thus can't fully discern the anatomy. Then, he probably can't perceive much of anything anyhow, due to his surroundings. Finally, it's by no means clear or specified that the natural-armored, whole-creature swallowing gullet even has a discernible anatomy even should the rogue be able to discern anything specifically, especially vital within the gullet itself. Sneak attack says: "The rogue must be able to see the target well enough to pick out a vital spot and must be able to reach such a spot." and I don't think that holds.
Secondly (and less conclusively), a sneak attack functions "If a rogue can catch an opponent when he is unable to defend himself effectively from her attack, she can strike a vital spot for extra damage." In general, a beast which habitually swallows creatures weapons and all, and can survive without serious problems a creature cutting itself out from inside of it, doesn't come across as one which is unable to defend itself effectively when it intentionally and in full awareness swallows a creature. A normal creature surely has a vulnerable gullet, but these specialized creatures which have a gullet so strong it can grapple others, with natural armor and capable of surviving holes, which is in constant contact with the creature at all times by the very nature of the situation makes it unreasonable to assume that they're particularly vulnerable when they swallow a creature or that their weaknesses are reachable from the very spot they most want to place a hostile creature in. A rogue requires that particular vulnerability, requires that her opponent be at a disadvantage to be able to capitalize on that weakness to make a sneak attack. Mechanically, rogues can only sneak attack those that are flanked or that have lost their Dex bonus. Clearly both of these circumstances don't apply.
It deserves extra mention why it's clear that a gullet's absence of dex modifier is not at all the same as being "denied a dex bonus". A creature that has a negative dex modifier, for instance, does not take it into account when determining its gullet's AC, but does take it into account when determining its flatfooted AC, so clearly the concepts are not strictly equivalent. Further, notably all cases in which a Dex bonus is denied, the term denied features prominently, and specifically, only the bonus is lost, never any penalty. A gullet's lack of dexterity bonus (by reason of lack of modifier) no more allows sneak attack than an average human;s lack of dexterity bonus (by reason of average dexterity).
Thus, there are two prerequisites which must hold before a rogue can use sneak attack, and there are multiple reasons to doubt they do. Taken separately, not all of these doubts might each individually conclusively demonstrate that sneak attack does not apply, but as a whole, it just can't be ignored. But more generally, pretending a gullet is functionally no different from an external limb in terms of mechanics will inevitably lead to absurd consequences. Since many rogues will be blind within a gullet, one could presume the gullet has total concealment. Yet concluding the rogue might miss it completely (due to concealment) is absurd - what else could she connect with? If a creature within a gullet attempts a ranged touch attack, say, by means of scorching ray, what AC must they hit? If they miss, what are they hitting? If a creature with swallow whole also has Evasion, can it evade a fireball going off in its gullet on a successful save? For that matter, can it succeed on any reflex save against an effect applying from within it's gullet? Does damage reduction apply to (non-elemental) damage caused by the gullet? Can a being grappled by a gullet be hit by those not being grappled? There may not be one right answer for all the above questions in all situations, but surely they clearly demonstrate that being grappled by a gullet is not merely the same as being grappled by some external limb - and once you make that distinction, why not critically examine sneak attack?
So...
I don't think a game will grind to halt if you allow sneak attack within a gullet, but I don't think it applies or makes much sense. Treating a gullet as if it were nothing special isn't an appropriate precedent to set since there are so many other ways in which it differs from "just any other limb".
Put another way, my player's would cry foul louder if a swallowing creature uses Evasion to evade an internal attack than they would if I were to deny sneak attack - thus, implicitly, they too don't consider the gullet to follow all game mechanics as written.
