The OQO Model 2+, BenQ S6, and Willcom/Sharp D4 all just got off the Matterhorn Bobsleds ride, which Disneyland describes as "turbulent" because of the sharp turns and sudden drops. All three units were on during their time on rails . . . and all three survived. They were doing something different when they got out of the bobsled cars than when they got in (the jostling caused some accidental button presses and application launches), but no harm was done.
I wouldn’t recommend using a MID or ultra mobile PC while on a ride, but that word of caution may not mean too much coming from someone who brought three of them to Disneyland today.