CNN has a photo of Bush managing to fall from a Segway (yeah, the thing was designed to make it impossible!): The Segway went down on Bush's first attempt, but he stayed on his feet with a flying leap over the machine. Undeterred, he got on again. His father climbed on a second Segway and they cruised around the driveway at the estate at Walker's Point.
They don't say it, but I know why it could ride it the second time: he swallowed his gum when he fell down! ;-)