N900 is not a "magical phone". If you don't know what you are doing then you shouldn't have bothered installing nitdroid in the first place. Blindly following instructions and not knowing specifically what each command does let alone understanding how linux and/or package managers in general would lead anyone to think N900 is some sort of "magical device". There is no magic when it comes to N900. A simple solution I would *highly* recommend is for you to backup and reflash. Maybe you can and will learn a lesson from all of this experience. It would be far quicker than trying to fix up any issues you have and to see you thread hijacking.

nitdroid has their own support forums, its called http://forum.nitdroid.com. There's plenty of nitdroid threads scattered on both this forum as well as nitdroid forum. So there's really no need to go hijack other people's threads when they were asking about [A] (or specifically apt-get cert issue) and you're trying to divert the case to be [B] (specifically nitdroid issue).

Now... looking back at what you have said:
Prior to this you have not installed nitdroid, am I wrong? This clearly shows you used dpkg to install nitdroid. Do you see the path that it clearly show:
Doesn't that tell you that you have installed nitdroid under "/and"? This is why:
Would not work because you can clearly see:
I don't know how you manage to go from 2.6.28-07 to 2.6.28-06. Nonetheless:
clearly shows:
So that now leaves two nitdroid related packages installed. One being "nitdroid-installer" package which needs to be manually removed. The other is "nitdroid-kernel-2.6.28-06" package which will require you to mount /and partition. Without you telling us exactly which guide you followed nobody can help you.

Hopefully you didn't continue on following the nitdroid installation guide to the point where you have basically FUBARed your maemo setup. Otherwise it will be a simple matter of just suggesting to backup and reflash.

In the meantime, I strongly urge you to start reading manual pages. In most linux installations, the manual pages are always included. The only exception are under embedded devices (including maemo). The only way to read manual pages if you cannot access one is via searching with Google. For example, to learn what dpkg is and how it works the normal command would be "man dpkg". If you don't have manpages installed anywhere else you can simply "man dpkg" into Google search and the first few links will be the links directly from the manual itself. There are few other minor exceptions but the bottom line is that you need to stop thinking N900 is some sort of "magical phone". It isn't, you just need to learn how it works (ideally having linux installed elsewhere and you constantly use it would be a bonus) is the most important step.

"alex" on nitdroid forum has said this before:
I personally wholeheartedly agree.