I encounter this idea regularly in stories, both submissions and published:
A character is bound (by a vow, by brain manipulation, by programming, by magic, whatever) to always tell the truth.
But most of the time when I see this, the author doesn't appear to have thought it through. The characters generally see this as a completely unbreakable guarantee of absolute truthfulness; it's rare that anyone in such a story attempts to explore what it might mean.
Start with the old question “What is truth?” These characters generally aren't omniscient, so they can tell the truth only insofar as they know it. But that's usually taken as given.
But there are plenty of further complications. Is withholding important information telling the truth? What about withholding information that you don't think is important but that is to the other person? What about withholding information the importance of which you're not sure about?
And what about misleading phrasing? If you ask “Is anyone else here?” and I decide to misleadingly interpret “here” as “within one foot of you and me,” and I say no, then am I telling the truth?
Then, too, language is inherently ambiguous, and communication involves both a communicating person and a recipient of the communication. If you tell the truth, the best will in the world on both our parts doesn't help if I interpret your statement as meaning something other than what you meant by it.
There seems to be this idea—comforting to those of use who like rules—that truth is binary; either something is The Truth or it isn't. But in the real world, truth is slippery, and there are shades of gray, and communication is hard.
All of which is fine if you're not messing with metaphysics. If someone has vowed to tell the truth in the real world, they're really just promising to do the best they can—and they might end up breaking that promise anyway, either intentionally or otherwise. But if someone in sf is bound to tell the truth, it's usually in some metaphysical sense—they are literally incapable of not telling the truth. (Maybe it's a robot that's programmed to tell the truth; maybe it's someone under a magical spell that, through whatever mechanism magic operates by, cannot utter untruths; maybe it's someone who's made a vow that has consequences, and if they break that vow then lightning will strike them dead. Or whatever.)
And because the very laws of the fictional universe are forcing the truth-teller to tell the truth, usually everyone else in the story takes everything the truth-teller says as unquestionable Truth.
And I just find all of that implausible and simplistic. I don't believe that there's such a thing as The Truth in the first place, but even if there were, the abovementioned caveats and grey areas make it seem implausible to me that truth-telling can be metaphysically enforced. And it seems to me that most people are likely to be aware of the inherent difficulties with the idea of truth-telling, which makes the other characters' unquestioning acceptance also seem implausible to me.
Of course, this is all yet another genre convention, and as such it's probably not subject to the kind of criticism I'm giving it. If it works for your story to have an oracle or a robot or a monk who isn't capable of lying, then go for it. Most readers and writers don't seem to have a problem with any of this; it may well just be me.
But for what it's worth, it almost always bugs me.
See also my 2003 blog entry Three Laws for the Robot-Kings—not the same topic, but related issues.
(Entry written in August 2012, but not posted 'til now.)
(See also Facebook thread for this post.)