When I turn on my N800 it shows the white screen with the Nokia logo, and just stays like that for around 30 seconds before turning off. No progress bar.

Have attempted to flash, enable R&D mode, etc using flasher-3.5 but in all cases the flasher won't recognise the device ("Suitable USB device not found, waiting.").

It's not a PC/software/cable issue as I can successfully run the flasher with -i on another N800 and that works fine (pressing home while turning on, after starting the flasher and then plugging in USB).

I never get the USB logo when I turn on while pressing the home key.

Have tried removing the battery and leaving it for a while before trying again.

The battery is fully-charged - charged on a known working N800 first. I have also tried with the battery of a known working N800.

Memory cards all removed.

Any suggestions on what to do next? Or is it dead?

Did you ever get your N800 working again? Last night mine rebooted on me suddenly twice in a row and then wouldn't turn back on. This morning I tried charging it and then turning it on. I get the white screen with Nokia logo that won't turn off. I haven't tried flashing yet, but have tried another battery and removing memory cards without any luck.

Happens to me every now and then. Try flexing the case firmly but not to hard. Put your four fingers on the top edge, and thumb on bottom and flex that back black cover a bit as there seems to be some kind of connection that gets shorted out or something. try that a few times as it always works for me, especially after it's in my pocket and the casing may get flexed like that. Good luck ;-)