With the help of the following steps, you'll be able to explore and tweak all system files and do some intersting hacking, like the one showed yesterday especially, to be able to access /usr/ directory :

1- You must be connected to a wireless LAN with your N900
2- Go to the application manager of N900, search for ssh server (and client, but this is optional) and install it
3- ssh on installation will ask you for a password (choose one)
4- Go to the application manager of N900, search for home ip (a widget on the desktop that always displays the n900 ip) and install it *
5- Go to your main PC and download a free ssh client with a graphic interface (it's much easier to use for a newbie) like winscp.
6- open winscp and login to the N900's as following :

Host name : N900's ip address that you get from home ip
Username : root
Password : your password (chosen in step 2)

6- You're done ! You have now the access to the /usr/ directory and you can explore and edit whatever you want (stay tuned for the upcoming "how to")

* You can replace the above step 3 with :