Nokia released a new firmware for the N800 tablets, and I flashed mine yesterday. It was by far the easiest upgrade ever.
I took a backup first and stored it on the internal flash card, and flashed the unit as per the instructions. One of the first questions asked was if I wanted a backup restored and I picked the backup made just before the flash.
The backup restored my gconf settings, and all the application repositories I had added, so while the non-standard applications weren’t restored automatically, all it took was to go to the Application Manager and install the ones I wanted. Applications that use gconf for user preferences had their preferences set already so I had to do nothing. For example, the FM radio applet had all the same stations as before the upgrade.