my user-name was on all Unix-systems (WEGA, SUN-OS , HP-Unix , Linux ) "halt" (pay attention : halt is my family name !!! ) This usname will not be shown on login-menü. After selection "OTHER" the username "halt" and the passwort can be writen, login is successfull. If an second user is createt on openSuSE 11.4 + XFCE, only this user can be selected by mouse. User "halt" is suppressed. Often the login-procedure (for user halt) must be done 2 time. The first try finished without public message .
The login screen (ie display manager) is not part of xfce. This is a bug in gdm/kdm/lightdm or any other display manager you might be running, but that's not the session manager.