I can confirm that when only having xbuntu-installed on feisty (installing if after a server install) you don't have the options menu with the greater which therefore prevents you from doing XDMCP logins like you can with the gnome login greater.
I can confirm that when only having xbuntu-installed on feisty (installing if after a server install) you don't have the options menu with the greater which therefore prevents you from doing XDMCP logins like you can with the gnome login greater.