I was a passenger in a 2005 Prius going at ~65 mph when the driver tried it. It didn't work. Needless to say, you can switch into neutral when stopped, so I gather there is some speed threshold above which it doesn't let you switch into neutral.
Edit: this made me curious, so I did some cursory research and found this:
According to one commenter, to shift into neutral when driving you can do one of the following:
1. Press the park button
2. Shift into reverse
3. Hold the shifter in the neutral position for 3 seconds
A video posted by a different commenter shows the driver holding the shifter in the neutral position for not quite 3 seconds, but still longer than is required to shift the car into other gears (and if my memory is serving me right, longer than is required to shift into neutral when stopped).
In any case, the most obvious way to shift the car into neutral did not work for us, and it's unlikely a panicked driver would think to try any of the methods listed above.
The gearshift in the Prius is totally electronic, and it does not allow you to switch into neutral if you're traveling above a certain speed.