The priuschat reference given above lists a lot of other good reasons why coasting in N may not be such a good idea. However, fear of damage to the power train is almost never a concern. If you are already travelling over 42 MPH when you pop into N, the gasoline engine will already be spinning. It will be quickly configured to idle, thus will continue to spin even without help from the electric motors. This will avoid any possibility of overspinning the smaller motor/generator. If you are under 42 MPH when you pop into N, it is unlikely that you will end up over 42 MPH. However, on those rare steep hills where this is possible, you could very well end up overspinning the smaller motor/generator. If you're lucky enough to have a 2004 or later model Prius, the smaller motor/generator is actually rated at 10,000 RPM instead of 8,000 RPM, so you have to go quite a bit faster than 42 MPH before there's a danger of overspin. If the 71 MPH number in the priuschat reference is correct, I really doubt anyone is going to find a hill steep and long enough to go from under 42 MPH to over 71 MPH in N.
But there are some wierd places on this planet, so I won't claim its impossible to find such a place.
