I should probably write a take on the gay Dumbledore thing.
I squeed. I giggled maniacally, and I beamed for an entire day. I am, in fact, 100% on board with this method of revelation.
Sure, JK could've written it somewhere into the books (a chapter heading in Rita's book would've been easily glossed over by Harry, for instance, but seen by the audience) and I would've been ok with that. I probably wouldn't've thrown my book at the wall after reading the Snape/Lily thing (seriously, I'm considering Veela!Lily canon. It's either that or Mary Sue, and I prefer my pink sunglasses here), rather, I could've forgiven it anything.
But I think that if she had written it into the book, then Gay!DD would be THE spoiler, and basically the ONLY thing anyone would ever focus on after that (well, it's not like she gave us much else in DH even remotely interesting), and it would definitely have taken Harry's thunder and then some.
Of course, it would've done a hell of a lot for gay acceptance, but I think that unless she revealed the information perfectly in DH, that is, subtly, with absolutely NO impact whatsoever on any character's opinion of DD, or any hint that his homosexuality had ANY bearing on the plot, the answer to the kid's question was the best alternative.
Because you see, anything other than the perfect mention of gay!DD would've turned him from "Greatest Wizard of All Time Liek Whoah also a God-like figure and all around beloved character" who happens to be, btw, queer as a three dollar bill, to, well, Big Gay Al(bus). And no matter what, it would've detracted from the (albeit shitty) plot, which I assume is a big no no for JKR.
Now, regardless of whether it was intentional on Rowling's part or not, the "announcement after the fact" method probably works best in this situation. It gives the audience a few months to revel in the awesome that is Dumbledore (whatever, he was probably one of the most interesting characters in the books despite (or because of) his dickishness), think, "wow, what that Dumbledore is some fella, rock on" and basically be generally on good terms with the character.
Then JK can drop the bombshell, and announce o hai, ur sweet d00d? Totaly gai.
Which means two things: the more intelligent people are going to evaluate their view of the gay, even if subconsciously, and also, while temporarily DD shifts to Big Gay Al, after the hubbub dies down, I fully expect his character status to drop back down to "GWoATLWAAGFAAABC" who happens to be, btw, queer as a three dollar bill.
So, in summary, I LOVE MY GAY HEADMASTER, AND I LOVE MY AUTHOR WHO MAKES ONE OF MY SHIPS CANON(ish) OMFGYAYWOO!!!!!
*pulls out list, scratches out "Dumbledore/Grindelwald"* Ok, only several hundred more pairings to go, world. Get on it.
ETA: Something interesting I just thought of while perusing meta: outing Dumbledore in an interview other than a book also does something else I quite like. If she'd outed DD in the books, it would've immediately skewed our opinions - the one gay romance was tehEVOL!! It sort of parallels gay with evil in the subconscious. However, everyone's already read the book sans gay, and the evil thing worked the exact same way while the readers percieved DD and GG as just really good friends. Adding gay changes absolutely nothing in the original books - essentially, Rowling is saying that it DOESN'T effect the plot, and I for one think that that's a good thing, considering the plot line it would've effected.
There will always be assholes who's opinions are suddenly skewed to hell due to this new knowledge, but I believe that most people will be less inclined to equate gay with bad, dead-end, and ultimately genocidal romances with Hitler since it's not in the books.