Hmmm.
Thats kind of funny. I have a 3rd gen iPod that I jog with almost every other day. I run about 7km in 20 minutes so its pretty frantic. I do run while holding it in either hand though.
I've dropped it more than 5 times and brought it jogging while going through a 160kph wind/typhoon trek around the village with pouring rain (I like the strong rain
)
So far the worse thing about it is that the battery lasts less than 2 hours on a full charge or overnight charge.
My guess is that because the drive plates are so tiny, the shake produces minimal disturbance. Think of a plate spinning on a stick. Big plates will wobble a lot while putting a coin on the stick will produce something pretty stable (or is it just unnoticeable?).