I was SO happy when I heard this, but not suprised, to be honest.
Lol, apparently he was in love with the dark wizard Grindelwald (sp?) much in the same way that Bellatrix was in love with Voldemort, but Dumbledore was different because he overcame it when he killed Grindelwald in the fight for the elder wand (hahaha, yessss I know

). All in all, it was Dumbledore's love that was his downfall. Grindelwald betrayed him, and broke his heart.
Even though Dumbledore being gay wasn't much of a suprise, I would have thought Percy, if anyone!
P.S.- much agreed about the whole "it wouldn't be a kids book if he obvious" subject.