There is a very good discussion about mainstream respectability for the fantasy and science fiction genres, over at SF Signal, which you can find here.
There's a lot there to absorb, but I agree that the mainstream doesn't actually exist anymore. Bookstores don't treat our genres as mainstream, keeping them in the so called SF and Fantasy Ghetto. But I seem to notice, from the perspective of far too many years, that the ghetto has grown considerably. Those SF and Fantasy shelves are generally much larger than they used to be, and often placed better in the stores.
At the same time, fantasy and science fiction books such as Harry Potter, Twilight, and a host of others continue to be racked in other sections, perhaps so that they won't get the 'taint' of SF and Fantasy on them. But that taint is dying, my friends. It's barely noticeable any more.
And that's all beside the point, since I don't yield the power and authority to bookstores to make that decision. They may be clinging to an outdated idea of what constitutes the mainstream, while SF and Fantasy stories dominate all other media.
Is there a mainstream, and if so, does SF and Fantasy finally, at long last, occupy a respected place in it? Who cares? Figuring out such things is what the fussy fiddlers do to occupy their time, the same way they worry about what is Art and what the themes of any given book might be. It doesn't matter. Whether we occupy a metaphorical sidestream or the metaphorical mainstream, telling good stories to a readership matters, and though I can't say we're doing as well as we can in that regard, we aren't doing so bad either.
At least I'm not doing so bad. I've made a living (sometimes a good one, sometimes not as good) telling stories for 25 plus years. A majority of my closest friends do the same. To quote Snoopy (from the musical, don't you know): Not bad. Not bad at all.