I've successfully flashed my N800 with NITdroid. I'm a newbie in the Linux world so I will offer a few of my findings and mistakes so that others might find useful:
*one problem with open source stuff is lack of documentation or I should say thorough documentation.

In the nitdroid_complete.sh:
"echo NOTE: This program should NOT be run as root.
echo It has specific parts that need to run as root."
What does this mean? confusing huh? What I found out is that... I'm logged in as my own username, but the shell script uses sudo (so the user will be root after the sudo command). There's this $USER variable that refers to the current user. So essentially I'm running part of the script on my username/Desktop while other stuff is running on root/Desktop, one way or another. An easy fix is to replace $USER with your current username in the .sh file. Save and run as: sudo ./nitdroid_complete.sh so everything will be in your own home / Desktop folders.

I have this N800 from the early days so the internal SDK card (the one besides the battery) has only 128MB of memory. I thought I should flash/install the OS into the external SD card since the internal one is less than 512MB. So that's my mistake #2.
Make sure you use the internal SD w/ at least 512MB of RAM and remove the external SD. My internal SD shows as /dev/sdc1 and the external shows as /dev/sdd1 if plugged in.

First boot of android, showing a few force close exception due to MMS, Alarm, etc.
Then black screen. Took out battery and reboot, much better now. Wifi works like a charm. However, the soft keyboard crashes all the time. So install this one instead:
http://code.google.com/p/netgents/

Now I got some hacking to do on it, besides my G1... ^_^