After trying Android on my N800, I decided to go back to Maemo (using this tutorial).

Using flasher-3.0, I could sucessfuly put Maemo back. The problem is that N800 won't perform that soft start with blue led slowly turning on while pressing power button anymore: When I push the power button, it immediatly turns on the screen with some green colored informations like kernel version and initf version and a USB icon (the same icon of flash mode).

The boot itself is ok and the rest is fine (Maemo is loading ok after that). The problem is that whenever anyhing touchs the power button, the N800 powers up, and that's not good (It powered itself this way during a flight, with all the wireless resources on: bluetooth and an ad-hoc wifi).

I haven't tried to flash using windows app because I don't have any PC running it, but I really think it's the same of flashing from Linux (ubuntu).

So, any ideas to really revert the boot back to factory style?

Ok, maybe I did'nt dig too much and I'm really sorry. I have just found that this is the "R&D mode". No one ever related this term to basic words like "flash mode" or "green letters". So, even knowing I will probably find my solution searching for "turn r&d mode off", I think this post will be "newbie friendly" for the future.
Now I'm looking for "turn r&d mode off"...

Now, I know nothing.

Humm, it seems that flasher for Linux has a lot of options, including --disable-rd-mode. And the Windows flasher (Nokia firmware updater) is pre-configured to turn r&d mode off. So, maybe I shoud have flashed using Windows in the first place.

Yes. It worked. No need to reflash, just did a "sudo ./flasher-3.0-static --disable-rd-mode". Hope this helps someone in the future. Otherwise it would be pathetic.