During today's live webcast from the Royal Albert Hall in Britain, JKR gave us a few more tidbits about OotP. Stephen Fry (who narrates the British audio books) was the "moderator" and asked questions from readers around the world. He also accidentally slipped up and spoiled the death!! If I hadn't already finished the book, I would have been quite mad at him after that! Oh well, it can be forgiven, I guess.
Anyway, JKR answered a question about why Snape is never allowed to teach Defense Against the Dark Arts. JKR answered with pretty much the same thing we have speculated here on these forums...when Snape was first hired by Dumbledore 16 years ago, he wanted to teach DADA, but Dumbledore felt that might "bring out the worst in him," so he told Snape to teach Potions instead, and has never felt ready to promote him.
Another reader asked why Harry never saw the thestrals before now, since he witnessed his parents' death. JKR kind of took this question and twisted it a little bit to assume the reader was asking why Harry didn't see the thestrals at the end of GoF, after he had witnessed Cedric's death by saying that it takes time for the death to fully sink in before you can see the thestrals. She pretty much admitted it was a cop-out, but that she did it knowingly and with the good intention of not wanting to confuse readers by introducing these mysterious black horses right at the end of GoF without offering any explanation.
I'm sure there were other things mentioned during the webcast that I've forgotten. It's going to be available to watch until July 3rd, so everyone should go check it out at http://www.msn.co.uk/liveevents/harrypotter/event/ before it's gone forever!