The Sheevaplug ships with ubuntu 9.04 and a kernel in flash. I wanted something more modern, and it turns out that Debian guru Martin Michlmayr has already done all of the work...
upgrade the "u-boot" firmware on your device to 3.4.19. All this takes is setting up a tftp server (I used tftpd-hpa) and copying u-boot*nand.bin from the upstream zip file into /var/lib/tftpboot - no DHCP hacking, just make sure you have an IP address to use for the brick itself...
/dev/sda1 bootable, and mount it on /mnt
debootstrap lenny /mntchroot /mnt
from inside the chroot, install more stuff as desired, but in particular, flash-kernel and linux-image-kirkwood
/etc/hosts /etc/passwd and any Kerberos or ssh host keys you needmodule-assistant, then later you can m-a update and m-a a-i openafs
/usr/sbin/flash-kernel and hard code machine= to the value it expects for the sheevaplug, then run it/etc/kernel-img.conf per the flash-kernel READMEAt this stage you've got a brick that boots Debian entirely off an external hard drive, and with the flash-kernel hook installed, allows you to treat it as an entirely normal Debian system, nothing special at upgrade time. I've got one set up as a kernel and package builder (the DNS server one is still running the stock ubuntu with security upgrades.)