If I manually erase all my content from my N900 (without using any flashing software), is it still accesable via the X-Terminal even though I can see no trace of any personal info on my phone (personal info such as pics, docs, contacts, calendar, passwords used on websites, etc?)

I'm assuming that by "manually erase", you mean "delete through the graphical file manager"...
The answer is probably not. To clarify: The terminal gives you full access to the filesystem of the device, while the File Manager shows you a slightly abstracted view of /home/user/MyDocs. If you delete a file from the File Manager, it should be gone. However, there are cases when certain information is stored or cached in places that are not accessible from the file manager. For example, some apps will use a thumbnails cache (usually in /home/user/.thumbnails), so if you delete photos, but do not use the terminal to wipe the cache, it is trivial for someone else to find out what at least some of the photos were. (Note that these are only very small thumbnails, not the original photos, but large enough that the content can be easily determined.) Other similar cases to this may exist, on Maemo and desktop Linux, such as "Recently Used" lists and other information or files that may stick around.
If you know your way around an OS, you can clean up this sort of thing manually in the terminal fairly quickly, but of course, you might miss something. If you want to make certain that your phone is clean of personal info/files, you really should reflash.
Another thing to note is that deleted files can often be recovered, and if someone had physical access to your phone, they might be able to salvage some of it. If it is on the FAT partition, you may have luck with secure-deletion programs, such as the command line program "shred", but no guarantees.
In general, if privacy is a concern, better overkill than not enough.

Ha ha, what have you got on ur phone that you need to be so sure its gone

I think when its gone its gone but im no linux geek, i supose the only way to be really sure would be to flash the emmc,
just back up what you want to keep reflash and then reinstall the (unoffending) files.

Maybe think outside the box and google or power search "recovering deleted files" will tell you something.

Edit: Looks like i type too slow, the above is a much better answer