First, someone asked Ron what he thought about fans' theories that Book was once an Operative, and he basically just didn't answer. Adam was like "I think you can tell them now. What is Joss going to do to you?" But Ron still didn't answer, just sort of smiled and went onto the next question.
Then, later, someone else asked if they had been in charge of Serenity instead of Joss, what they would have changed about what happened with their characters. Christina said, "Well, I would have been in it.." Aw. Adam said he'd be "homesteading with Inara.. he really liked Inara." Ron first said "I'd be Nathan!" and someone said "Isn't one enough?" He laughed, and then seemed to be pondering and someone called out "Alive!" And he laughed at that too. Then he said something about "I kinda would have liked to have been an Operative, but.. hm, done that!" OOOOH. Could that be confirmation of the theory? Most people who were at the panel seem to be thinking YES.