Well, it was more of a reception at a Sci-Fi Con, actually. A friend of the family is one of the conveners of the big annual Con here in KC, and they had Mr. Niven in a few years back. I got to go for free due to my association with the friend, and also got some preferential seating in the deal.
I had a million things I wanted to ask him, but Larry was too intent on getting laid by this long-haired plumpish vixen that was there. My, but he was paying a *lot* of attention to her. I imagine he does pretty well at these fan flings.
I'll try to dig up my autographed copy of The Integral Trees and scan it for you...and yes, he did write Ringworld. It was the first Niven book I ever read. I started reading it one night at bedtime. I had to be at work at 6:00 AM the next morning. At 3:30 AM, I picked up the phone and awakened the friend of mine who'd given it to me to read, and yelled, "You son of a bitch! You gave me this fucking book and now I can't put it down, and I have to be at work in two and a half hours!"
As to what he was like, well...he was like this middle aged, really intense, quiet guy who only seemed to brighten when he was trying to get laid. He wasn't real interested in talking. Not stuck up or arrogant or anything, more real, real internalized.