I have created small update for Kindle 3. It make File Sytems backup (excluding userstore) and places it into Kindle root directory (Userstore).
http://www.audi-bel.com/kindle/update_backup_0.01.zip
Backuping can take ~10min. Backup size will be near 700Mb so you must have free space on userstore.
P.S. Restore can be done via Serial line