I discovered the following.
If you open the menu item "connect to server" and write localhost, you will be connected to the same machine on which you are working. You will asked the user for the login. If you choose a user with administrator priviledges, you will asked to connect either to the volume of that user or the to the entire volume.
If you choose the entire volume, you can navigate over the full set of directories of the volume. This means that you will be able to see also the classic Unix directories such as /etc, /usr, /tmp etc. In this manner, you can use the GUI to navigate to the hidden directories on MacOsX.
Just one warning: inside these directories, there are some UNIX links that are displayed as folders even if they point to normal files. If you click over some of these folders, the Finder seems to go to an infinite loop. In this case, in order to regain the control of the machine, press the button on the keyboard used normally to startup or shutdown the system. It will apperar a pop up window that will ask you to shutdown the system or to put it in "stop" mode.
If you choose "stop", the system will break the connection to the localhost server and then stops. Then, pressing a button of the keyboard, the system immediately restart without loosing anything except the freezing connection. You will be able to continue working normally.
[Editor's note: Interesting tip; I haven't yet tried this myself, but it seems like an alternate method to get to the hidden UNIX folders quite easily. Just watch that infinite loop caution!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010806070635722