Install backupmenu.
Turn your n900 off.
Open the keyboard, keep it open, turn your n900 on
Use the menu to make a backup of both rootfs and optfs on your sdcard.
When you send your n900 to nokia care, keep your sdcard home.
When it's back and reflashed, reinstall backupmenu.
Turn off, install sdcard, open keyboard, turn on.
Choose restore from menu, choose your rootfs and optfs from sdcard.
Wait until done.