Anyone with a PowerBook who frequently switches network locations may have come across this problem in Mac OS X 10.2.1. When you are trying to switch locations, you suddenly you realize that ALL of the locations you have set up are now gone! What's more strange with this problem, is that when you try to recreate the locations you had before, they do not save anymore! I'm not sure what causes this problem, but it appears to be an issue with the permissions getting switched on the "preferences.xml" file to "read-only"...
In order to cure the problem of disappearing network preferences in the Apple menu's Location submenu, I successfully performed the following instructions to repair my "preferences.xml" file. In the Finder, select Go -> Go to Folder. In the dialog box that appears, paste in /var/db/SystemConfiguration/ and then click "Go".
When this window opens, there is a file called "preferences.xml". The permissions on this file must have been changed by some software that you recently installed. To fix the permissions on the file, click on preferences.xml one time and do File -> Get Info. The Get Info window will appear; click on the "Ownership & Permissions" drop down arrow within the dialog. All of the options are probably grayed out if you are logged in as yourself. The "Access" option under "Group" has probably been changed to "Read" or else "No Access". You need to change this back to "Read & Write". To change this back, click on the lock icon so that it is unlocked. Change the "Owner" of the file to your login name. Next, change the "Group" to the group your user ID belongs to, mine was "staff (Me)". Next, change the "Access" drop down menu under Group to "Read & Write". When you're done modifying the permissions. Go back up to the "Owner" of file, and change this back to "System".
When you're done performing these instructions, close this window and try opening your System Preferences to add in some new "Locations" to the Network Preferences Pane. They should now add in, and be visible under the Apple Menu-> Locations submenu again! Woohoo!
[Editor's note: I haven't had this problem myself, but I have had others ask me about it; this is the first fix that I've seen. If you try this fix, please post a comment to let us know if it worked for you.]

