[Zoid]

There is no need to use the command line to edit your own preferences file, nor do you have to be root.

Any files in /Users/<your account>/Library are owned by you and you can freely edit them without becoming root.

Also, if you have the developer tools installed (which I recommend even if you are not a developer), you can use the PropertyListEditor to edit preferences with a graphical client.

Run PropertyListEditor from /Developer/Applications/PropertyListEditor and go to File/Open and type in the file name as stated earlier,
Users/your_username/Library/Preferences/.GlobalPreferences.plist

PropertyListEditor will then display the properties in a window and you can easily double click their values and change them as desired. This works for any .plist file in your Library/Preferences folder. It's also a great way to investigate various preferences files for different applications and see what preferences you and mess with yourself.

Of course, if you are experimenting, make sure to back up any plist files before you modify them! Also, remember preferences do not take affect until you relaunch the application that uses the preferences file. Also be aware that if you are editing a plist file for an application that is currently running, it may overwrite any changes you make if you quit that application after you save changes. In general, its best to edit preferences files when the application is not running.

Good luck!