After installing xubuntu-desktop on Ubuntu 10.10, The GDM window icon changed to a blue mouse, How to I return GDM to the default Ubuntu theme/icon.
I have tried the following which effectively changes the theme but doesn't modify the top icon.
http://www.n00bsonubuntu.net/content/how-to-change-login-screen-ubuntu-1010-maverick-meerkat
Install Ubuntu tweak which allows you to change it very easily.
the icon is located at /usr/share/icons/LoginIcons/apps/64/computer.svg
root can overwrite this icon with another svg item.
you could also place a second icon there and change the value of /apps/gdm/simple-greeter/logo-icon-name in the configuration-editor.
This works for me (Ubuntu 10.04): Do a left click on your username in the panel (usually in the upper right of the screen). In the menu do a left click again . This opens the window "About " where your logon icon (and more) can be customised.
HTH
Run login to a recovery console and run
gksu -u gdm gnome-appearance-properties
and change the icon theme. You could also google gdm2setup (Sorry I do not have a link handy).