I've always felt a kinship with Scott Westerfeld because my maiden name was Wester. OK, I know Wester and Westerfeld are not the same name, but hey, it's closer to my name than any other spec fic author I know. (For the same reason I've always wanted to go to Westercon, but I never made it.) Now I have the double kinship of also writing YA fantasy myself (as he does), so I say here's three cheers for Scott Westerfeld!
But also, it seems to me that YA literature isn't chunked up into "mainstream" or "contemporary" or "historical" or "fantasy," it's all just "books young people might want to read." Military science fiction author John Hemry (who also writes YA) says that to be YA, a story has to have a young protagonist and be about a life-changing event. This includes a boatload of stories, from To Kill a Mockingbird to the Gossip Girl books. You could say that makes it a genre in and of itself, or you could say that kids are less interested in labels than adults.