A post on the Flash Mobile blog claims that the impact of running Flash on the Google Nexus One is only around 6 percent, noting "Our own tests show that video can be played for well over 3Hours over WIFI from youtube in H.264." Gruber points out the bad math in the post noting that the Nexus One is rated for 7 hours of video playback time.
So, it's not quite the 6 percent penalty that's claimed, but it's certainly not as bad as Steve Jobs' says either. Jobs claimed that Flash video would reduce the iPad's battery life from 10 to 1.5 hours. I guess that the reality is somewhere in the middle.