There are several possibilities to install Debian on the Sheevaplug.

First of all the Kernel which comes preinstalled with the Sheevaplug would be enough to run a Debian System. So the first approach would be to create a root file system from scratch using debootstrap. This file system can then be copied over to a SD Card or USB Stick. The boot loader uboot would get a new config in order to start the original Linux kernel aside with the new debian root file system.

In favor to habe a fully debianized system on the Sheevaplug the next step is to add to the debootstrap process also the possibility to include a debian based kernel.

Here is a link about how to create a debian based root file system found on plugcomputer.org and here is another more sophisticated howto which also includes the newest debain kernel which also supports the Sheevaplug.

Doing the above approach means really to go down deep into building a Debian based system. It includes a lot of manual steps and requires a really good understanding of the Linux system. The positive thing about it is to learn a lot how Debian works and how it is build together.

The alternative and much faster approach is to use the "normal" Debian Installation, which recently was adapted for the Sheevaplug. Martin Michlmayr has a great howto for installing Debian stable and testing on the Sheevaplug here. Basically all you need is to download a Linux kernel (uImage) and init RAM disc (uInitrd), copy it over to the plug, install the Debian System normally like you would do on a normal desktop system and adapt uboot boot loader at the end to start always your new Debian.

My Sheevaplug now runs Debian on a 4 GB SD card and finally my homepage (which you are now reading) is running from it. :-)

Here a picture of my Sheevaplug just running under my cupboard.