I've flashed the N800 so many times I can do it in my sleep (systems programmer ;-) ).

The sequence I always use is:
- turn off N800 device. If that doesn't work, open battery compartment and remove battery (best removed by inertia) completely and then insert it again.
- connect N800 to PC.
- on the PC, call
sudo ./flasher-3.0.amd64, -F RX-34_DIABLO_5.2008.43-7_PR_COMBINED_MR0_ARM.bin, -f -R
- it will say something like "device not detected", that's normal since it's not switched on.
- switch the N800 on and then IMMEDIATELY HOLD THE HOME BUTTON.
- PC flashes to N800
- don't forget to disconnect cable after you are done.

(for some reason the Nokia people like commas in their filenames, that's not a mistake, the file is actually named like that)