In the comments of Welcome to Heaven, Wei Dai brings up the argument that even though we may not want to be wireheaded now, our wireheaded selves would probably prefer to be wireheaded. Therefore we might be mistaken about what we really want. (Correction: what Wei actually said was that an FAI might tell us that we would prefer to be wireheaded if we knew what it felt like, not that our wireheaded selves would prefer to be wireheaded.)
This is an argument I've heard frequently, one which I've even used myself. But I don't think it holds up. More generally, I don't think any argument that says one is wrong about what they want holds up.
To take the example of wireheading. It is not an inherent property of minds that they'll become desperately addicted to anything that feels sufficiently good. Even from our own experience, we know that there are plenty of things that feel really good, but we don't immediately crave for more afterwards. Sex might be great, but you can still afterwards get fatigued enough that you want to rest; eating good food might be enjoyable, but at some point you get full. The classic counter-example is that of the rats who could pull a lever stimulating a part of their brain, and ended up compulsively pulling it, to the exclusion of all else. People thought this to mean they were caught in a loop of stimulating their "pleasure center", but it later turned out that wasn't the case. Instead, the rats were stimulating their "wants to seek out things -center".
The systems for experiencing pleasure and for wanting to seek out pleasure are separate ones. One can find something pleasurable, but still not develop a desire to seek it out. I'm sure all of you have had times when you haven't felt the urge to participate in a particular activity, even though you knew you'd enjoy the activity in question if you just got around doing it. Conversly, one can also have a desire to seek out something, but still not find it pleasurable when it's achieved.
Therefore, it is not an inherent property of wireheading that we'd automatically end up wanting it. Sure, you could wirehead someone in such a way that the person stopped wanting anything else, but you could also wirehead them in such a way that they were indifferent to whether or not it continued. You could even wirehead them in such a way that they enjoyed every minute of it, but at the same time wanted it to stop.
"Am I mistaken about wanting to be wireheaded?" is a wrong question. You might afterwards think you actually prefer to be wireheaded, or think you prefer not to be wireheaded, but that is purely a question of how you define the term "wireheading". Is it a procedure that makes you want it, or is it not? Furthermore, even if we define wireheading so that you'd prefer it afterwards, that says nothing about the moral worth of wireheading somebody.
If you're not convinced about that last bit, consider the case of "anti-wireheading": we rewire somebody so that they experience terrible, horrible, excruciating pain. We also rewire them so that regardless, they seek to maintain their current state. In fact, if they somehow stop feeling pain, they'll compulsively seek a return to their previous hellish state. Would you say it was okay to anti-wirehead them, since an anti-wirehead will realize they were mistaken about not wanting to be an anti-wirehead? Probably not.
In fact, "I thought I wouldn't want to do/experience X, but upon trying it out I realized I was wrong" doesn't make sense. Previously the person didn't want X, but after trying it out they did want X. X has caused a change in their preferences by altering their brain. This doesn't mean that the pre-X person was wrong, it just means the post-X person has been changed. With the correct technology, anyone can be changed to prefer anything.
You can still be mistaken about whether or not you'll like something, of course. But that's distinct from whether or not you want it.
Note that this makes any thoughts along the lines of "an FAI might extrapolate the desires you had if you were more intelligent" tricky. It could just as well extrapolate the desires we had if we'd had our brains altered in some other way. What makes one method of mind alteration more acceptable than another? "Whether we'd consent to it now" is one obvious-seeming answer, but that too is filled with pitfalls. (For instance, what about our anti-wirehead?)
I imagine the ultimate wireheading would involve complete happiness and interfacing with the FAI's consciousness, experiencing much more than is possible by a solitary mind.