Globalscale has released a new Sheevaplug reflash guide:http://globalscaletechnologies.com/download/Sheeva_USB_Reflash_Guide_Upgrade.docx
I've used this to reflash several older Sheevaplugs, and it works well after some changes. The filesystem image provided (rootfs.tar.gz) does not have /dev/console, /dev/null or /dev mtd*. Create these first and the reflash works perfectly. This will install Debain (Kernel 3.13.10) to NAND to replace Ubuntu 9.0.4. If you have bad NAND blocks you can boot and use nandtest to mark them as bad before proceeding. I did not need to upgrade uBoot for this to work, just resetenv.