Look for the FAleaks threads on this website. He was more or less outed as such when a private confession from one of his victims was leaked. He's always had a certain tendency towards that kind of personality - he's a short Asian dude but plays a massively built dingo online, has trouble being nice to women and is generally douchey and needlessly aggressive online and off. And Jesus, look at his art. It's always his big huge character (again, IRL he's short and small) raping some smaller character. The size difference can be so ridiculous that people began asking if the females were underage. They're not, but it's just that ridiculous.
I believe there were suspicions and rumors about this particular incident but no real confirmation until the note leak of December 2010. A private confession from the victim to Dragoneer - asking for help - was leaked. She had no reason to lie, since the note was never meant to be public, and her account matched what little was known about the incident. Then Dragoneer says he won't help a sexual assault victim because popufurs, but that is another topic.
When the leak happened - Adam reacted like he'd been caught. He blacked out his online accounts and sort-of went into Internet hiding. After a few months he was back, though, and being the asshole he always was. Fast forward to the aftermath of Anthrocon 2012, and someone else has been calling him out over Twitter and FA journals regarding another rape incident - allegedly three guys involved in this one. If there's any more concrete info about this incident, no one's posted it here.