Getting Linux onto Your iPod
There are several open source groups working on porting Linux to the iPod. The most mature that I have found is iPodLinux. The folks over at iPodLinux have been focusing on porting a slimmed-down version of Linux to the iPod. The version they are using is uClinux, which is a specific version of Linux that can be embedded into devices. You can think of this as the same as Windows 2000 for embedded devices (turn on your Xbox, and you will see a version of it running). uClininux has some drawbacks, however (for example, multitasking is tricky at best).
The version of uClininux is designed specifically for installation from a Linux desktop. The full installation instructions are available here.
What you will like about this installation of Linux in your iPod is that it does not destroy the content you already have on the iPod. So you can get back to your original default settings.