Hi again! I've been monitoring my ~/.xsession-errors and couldn't find any problem but the disconnection of GNOME's session manager. As you'll see below, gnome-panel and nautilus report issues because of this. But, after continuous usage, also PulseAudio, gnome-power-manager and others also do complain.
Here's my ~/.xsession-errors at startup: /etc/gdm/Xsession: Beginning session setup... SESSION_MANAGER=local/UGI:@/tmp/.ICE-unix/5583,unix/UGI:/tmp/.ICE-unix/5583 1251281797.312432 Session manager: disconnected... Advertencia del gestor de ventanas: Ocurrió un error al leer el archivo de sesión guardado /home/ugi/.config/metacity/sessions/default0.ms: Failed to open file '/home/ugi/.config/metacity/sessions/default0.ms': No existe el fichero o el directorio ** (gnome-volume-control-applet:5773): DEBUG: Disabling debugging GNOME_KEYRING_SOCKET=/tmp/keyring-cC9VoN/socket SSH_AUTH_SOCK=/tmp/keyring-cC9VoN/socket.ssh ** (nautilus:5752): WARNING **: Unable to add monitor: No soportado ** Message: Could not connect to session manager: Could not get owner of name 'org.gnome.SessionManager': no such name ** (gnome-panel:5748): WARNING **: Could not connect to session manager: Could not get owner of name 'org.gnome.SessionManager': no such name (gnome-panel:5748): Gdk-WARNING **: /build/buildd-gtk+2.0_2.16.5-1-i386-FXZUi1/gtk+2.0-2.16.5/gdk/x11/gdkdrawable-x11.c:878 drawable is not a pixmap or window 26/08/2009 12:16:43 Autoprobing TCP port in (all) network interface 26/08/2009 12:16:43 Listening IPv{4,6}://*:5900 26/08/2009 12:16:43 Autoprobing selected port 5900 26/08/2009 12:16:43 Advertising security type: 'TLS' (18) 26/08/2009 12:16:43 Advertising authentication type: 'No Authentication' (1) 26/08/2009 12:16:43 Advertising security type: 'No Authentication' (1)