While it's not indestructible I bet these devices could take a pretty hard shaking. There are no moving parts inside and probably a cable connecting the battery to the mainboard and a cable connecting the screen to the mainboard. The rest is probably all integrated to the board.
You could use the movement sensor to count your rounds too, turn red when you only have 5 left and the like! I just mentally patented that iPhone app btw!
Actually in someones show show coverage there is a rail mounted device to count your rounds now, looks kinda like halo or something lol.
The ipod in that picture is mounted on a KAC M110 SASS. Knights armament made the mount and that ipod is Reed Knight III's personal ipod. I went to the SHOT show and i have friend that works for KAC.
There is another app called iSnipe that provides similar data for just a few bucks. I think it could be pretty damn handy at a 3-gun match that had some long shots. The key to any of these is having a rock-solid zero, knowing it and knowing your optics.