The Readercon blurb talks of "stories in which its unclear whether the fantastic element is real or imagined by the characters". Personally, I find that one of the less satisfactory forms of ambiguity. For one reason, it's hard to tell whether the ambiguity is real or imagined by the reader. Take, for instance, The Return of the Screw. No one thought there was any doubt about whether the ghosts were real for decades (and suspicious minds will note that the critic who first doubted it was Edmund Wilson, whom you may also know as the man who reviewed The Lord of the Rings, and the review was titled "Oo, Those Awful Orcs").
Or Neverwhere, which the blurb cites. There there is more evidence -- Richard is explicitly told that he is having a nervous breakdown at one point and that all he is seeing is a hallucination. On the other hand, this is while he undergoes an ordeal. . . but the point that I think the ambiguity founders on is that when he returns to his normal life, he's received a promotion. Not something that a nervous breakdown would explain.
And worst of all, I was in a bookstore shortly after the release of Harry Potter and the Order of the Phoenix, two clerks were talking about the scene where Umbridge falls afoul of the centaurs, and one customer said that she believed that the last book would reveal that the entire series had been a fantasy of an abused boy.
Which is why I cast a jaundiced eye on such interpretations unless there is evidence of their accuracy. Then, for the writer, that gets really fun. How do you make it clear in the book that things really are ambiguous?
My own favored form of ambiguity is where the characters as well as the readers are uncertain. One character thinks it can't be chance that he met another (preferably early in the book, to avoid annoying the reader), and the other is contemptuous of the notion. A magical item is supposed to produce an effect, and the effect happens, but to all appearances, naturally. Things fall off shelves, but only when they were already near the edge, so there's no absolute need to invoke a ghost. For one thing, it's much easier to make ambiguity clear when your characters can argue over it. . . .
part of bittercon