Aside from what DanielMartin said, there isn't much you can do unless you have, say, openssh-server installed. If you can navigate the terminal blind (I can), then you can do a lot more, but I'd imagine if you could do that, you wouldn't be asking us...
If you tap the next button up from the PC Suite button (on USB connect), then you can grab all your files. Unfortunately, the N900 exports /home/user/MyDocs over USB rather than /home/user (presumably so newbies don't trash all their settings by accident, wondering what the .dotfiles are), so you can't directly access things like contacts using Mass Storage.
Good luck!