Actually back on June 1st, 2010. I was on my way back from Japan on JAL flight 007 on their 777-300 ER and the jet had a top speed of 820 MPH. I was shocked when I saw that speed on the seat mounted TV I expected the Captain to make an announcement, however he never did, and I could tell we were in the jet stream due to layered clouds well below the plane at the extremely fast rate of speed for that brief moment of time. Most of the time of the flight was the speed was <700 MPH.