I know that you can run Ubuntu from a flash drive, booted from another machine. What I'm wondering is whether it's possible to boot such an image from an iPad (jailbroken or otherwise). I know that apple doesn't want you to use a usb drive, but I understand that it's possible to attach a usb drive using the usb camera adapter.
There have been projects to get operating systems other then iOS installed on the iPhone, however, it's not as simple as putting a bootable image on a USB and connecting it to an iDevice. For more information regarding this see:
There seems to have been a lull in development lately. The short answer to your question is that currently it's not possible to boot a Linux image from an iPad/iPhone.