I think JK came out and said it, because nobody had mentioned it. and I think it DOES MATTER, that the only person in Albus Dumbledore’s life to ever achieve a romantic link was Grindelwald, and Albus had to kill him. I think that is HUGE, to a story about friendship.
I mention Albus by name as the other Dumbledore, Aberforth Dumbledore, had his own sexual issues with goats, but that is probably for a different thread.
I have to admit that I don’t care one way or the other what the sexuality of the different HP characters is, but I wasn’t surprised to see her a) not include Dumbledore’s sexuality explicitly in the books and b) announce it later. I appreciate that she keeps providing greater detail about the world of HP, even if it’s outside the “canon.”
Though if she had wanted to be very progressive - that epilogue, people. Let’s pair everyone off into their respective heterosexual couples with children? That doesn’t really reflect reality at all. Dumbledore’s sexuality is a bit more realistic, in my mind.
(And no, I wasn’t surprised, what with the purple velvet suit he wore when he met Tom Riddle as a child (I know, stereotypical, but nonetheless) and hints about the depth of his close relationship wtih Grindelwald.)
Well it’d be kind of a shock if Harry showed up with Neville having no hint of it now wouldn’t there be? =P
I didn’t find the revelation that Albus was gay too surprising and I don’t care one way or the other about his sexual orientation…now Aberforth is another matter all together. 
And…
[SPOILER]Albus Dumbledore didn’t kill Grindelwald…just defeated him. Voldemort killed him in his prison cell trying to find the Elder Wand.[/Spoiler]
It’s happened already in /fanfic…more than once I’m sure.