The OpeniBoot project:
OpeniBoot is an open source implementation of iBoot for Apple iPhone and iPod Touch devices. It allows booting of unsigned code such as linux kernels on the device. There is also and OpeniBoot console (oibc) which can be connected to from a libusb enabled computer.
OpeniBoot is critical for the booting and usage of linux on the iPhone as it sets up and initializes many devices ready for the linux kernel to take over. Many iBoot functions are replicated here, however when booting iOS iBoot is chainloaded from the NOR to guarantee iOS will boot without problems even with OpeniBoot installed.
Pretty neat stuff going on MacGasm has a video of it in action. MacGasm reports that the project is hoping to get Android OS up and working on the iOS devices – which would be kinda neat.