Other than dual-booting Android on the iPhone, I fail to see any other credibly useful reason to run the Linux kernel or some Linux distro on an iDevice.
IMHO the hardware is quite capable. Meaning that it would be a very convenient headless server with its own UPS.
Similarly to custom ROMs for Android breath new life in non supported devices, I believe running Linux on an old iDevice would be simple way to get perfectly good hardware doing something again.
