Not sure on a flat sprint--haven't done that (and tried very hard) for a while. I hit around 58mph on a downhill in the mountains north of Atlanta though. It was fun, but now that I think about it maybe not too smart.
I just finally put a speedometer on my bike so I now have an answer. I was going 39 last weekend through the Presidio in San Francisco. I wouldn't really consider it my "personal best" since it was just gravity pulling my fat butt down the hill. Although, it was fun and rewarding after just riding the climb that took me to the top of that hill.