Just updated my N800 and my girlfriends N800. For her device it did work without problmes. Mine did end up in a reboot loop. :-(
Pretty much the same apps installed on both devices. Also only "normal" apps from Extras repository. No low level stuff like different kernel, screen rotation or something like that.

The only difference coming to my mind is, that on my girlfriends N800 only apps from the Diablo Extras repository are installed. On my N800 I had also some apps installed like Maemo Mapper which are coming from Chinook Extras.
Maybe that's the reason for the reboot loop?

Anyway, after a reflash I was able to successfully update also my N800...