After two days of flighting, I finally succeeded in restoring the nand with the combination of netbooting a small image (
http://mark.ossdl.de/2009/09/network-booting-linux-on-the-sheevaplug/comment-page-1/#comment-7288) and the restore described in the documentation "SheevaPlug - Writing Jaunty Filesystem on the NAND flash".
I am still interested in learning how to setup a net boot with nfs root for developement purpose. My initial net boots with that got same error as the USB boot, hanging after unpacking the kernel. Still not sure wht this happens...