I've just read through the NYT Book Review's Top 100 list for 2008. I didn't expect to find any science fiction or fantasy there, of course, especially since William Gibson has evidently stopped writing it. But--lo and behold--there's an odd Russian novel there called The Sacred Book of the Werewolf. This book has to be considered, by any measure, speculative fiction:
So you don't have to actually follow the link, I'll give you the gist: A girl who appears to be a sex worker is actually a werewolf with a sort of magical foxtail who hunts investment bankers by night . . .
My theory, having also read The Time Traveler's Wife and The Historian, is that mainstream readers secretly long for the fantastic, but are too embarrassed to admit it. So the industry sneaks a few spec fic books into its mainstream lists, and hopes no one will really notice! It's kind of like snitching your kid's french fries when no one's looking.