Fabrice Lorrain reported in Debian bug http://bugs.debian.org/467144
"""
Hello,

gdm/gdmgreeter works find but is reporting GLib-GObject-CRITICAL at
statup.

@+,
    Fab

$ grep gdmgreeter daemon.log | grep "11:31"
Feb 23 11:31:47 arkham gdmgreeter[6066]: GdkPixbuf-CRITICAL:
gdk_pixbuf_new_from_file_at_scale: assertion `width > 0 || width == -1'
failed
Feb 23 11:31:47 arkham gdmgreeter[6066]: GLib-GObject-WARNING: invalid
(NULL) pointer instance
Feb 23 11:31:47 arkham gdmgreeter[6066]: GLib-GObject-CRITICAL:
g_signal_connect_data: assertion `G_TYPE_CHECK_INSTANCE (instance)'
failed
Feb 23 11:31:47 arkham gdmgreeter[6066]: GdkPixbuf-CRITICAL:
gdk_pixbuf_new_from_file_at_scale: assertion `width > 0 || width == -1'
failed
Feb 23 11:31:47 arkham gdmgreeter[6066]: GLib-GObject-WARNING: invalid
(NULL) pointer instance
Feb 23 11:31:47 arkham gdmgreeter[6066]: GLib-GObject-CRITICAL:
g_signal_connect_data: assertion `G_TYPE_CHECK_INSTANCE (instance)'
failed

$ grep -v "^#" /etc/gdm/gdm.conf | grep -v "^$"
[daemon]
[security]
AllowRoot=true
[xdmcp]
[gui]
[greeter]
GraphicalThemes=industrial/:happygnome/:blueswirl
GlobalFaceDir=/usr/
DefaultWelcome=false
GraphicalTheme=blueswirl
[chooser]
[debug]
[servers]


-- System Information:
Debian Release: lenny/sid
  APT prefers unstable
  APT policy: (500, 'unstable')
Architecture: i386 (i686)

Kernel: Linux 2.6.22-3-686 (SMP w/2 CPU cores)
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages gdm depends on:
ii  adduser              3.105               add and remove users and groups
ii  aterm [x-terminal-em 1.0.1-3             Afterstep XVT - a VT102 emulator f
ii  debconf [debconf-2.0 1.5.19              Debian configuration management sy
ii  fluxbox [x-window-ma 1.0.0+deb1-5        Highly configurable and low resour
ii  gksu                 2.0.0-5             graphical frontend to su
ii  gnome-session        2.20.3-2            The GNOME 2 Session Manager
ii  gnome-terminal [x-te 2.18.4-1            The GNOME 2 terminal emulator appl
ii  konsole [x-terminal- 4:3.5.8.dfsg.1-7    X terminal emulator for KDE
ii  kwin [x-window-manag 4:3.5.8.dfsg.1-7    the KDE window manager
ii  libart-2.0-2         2.3.20-1            Library of functions for 2D graphi
ii  libatk1.0-0          1.20.0-1            The ATK accessibility toolkit
ii  libattr1             1:2.4.41-1          Extended attribute shared library
ii  libc6                2.7-8               GNU C Library: Shared libraries
ii  libcairo2            1.4.14-1            The Cairo 2D vector graphics libra
ii  libdbus-1-3          1.1.4-1             simple interprocess messaging syst
ii  libdbus-glib-1-2     0.74-1              simple interprocess messaging syst
ii  libdmx1              1:1.0.2-2           X11 Distributed Multihead extensio
ii  libfontconfig1       2.5.0-2             generic font configuration library
ii  libglade2-0          1:2.6.2-1           library to load .glade files at ru
ii  libglib2.0-0         2.14.6-1            The GLib library of C routines
ii  libgnomecanvas2-0    2.20.1.1-1          A powerful object-oriented display
ii  libgtk2.0-0          2.12.8-1            The GTK+ graphical user interface 
ii  libpam-modules       0.99.7.1-5          Pluggable Authentication Modules f
ii  libpam-runtime       0.99.7.1-5          Runtime support for the PAM librar
ii  libpam0g             0.99.7.1-5          Pluggable Authentication Modules l
ii  libpango1.0-0        1.18.4-1            Layout and rendering of internatio
ii  librsvg2-2           2.22.0-1            SAX-based renderer library for SVG
ii  librsvg2-common      2.22.0-1            SAX-based renderer library for SVG
ii  libselinux1          2.0.35-1            SELinux shared libraries
ii  libwrap0             7.6.dbs-14          Wietse Venema's TCP wrappers libra
ii  libx11-6             2:1.0.3-7           X11 client-side library
ii  libxau6              1:1.0.3-2           X11 authorisation library
ii  libxdmcp6            1:1.0.2-2           X11 Display Manager Control Protoc
ii  libxext6             1:1.0.3-2           X11 miscellaneous extension librar
ii  libxi6               2:1.1.3-1           X11 Input extension library
ii  libxinerama1         1:1.0.2-1           X11 Xinerama extension library
ii  libxml2              2.6.31.dfsg-1       GNOME XML library
ii  lsb-base             3.1-24              Linux Standard Base 3.1 init scrip
ii  metacity [x-window-m 1:2.20.2-1          A lightweight GTK2 based Window Ma
ii  mrxvt [x-terminal-em 0.5.3-1             lightweight multi-tabbed X termina
ii  openbox [x-window-ma 3.4.5-1             standards compliant, fast, light-w
ii  pterm [x-terminal-em 0.60-2              PuTTY terminal emulator
ii  rxvt [x-terminal-emu 1:2.6.4-12          VT102 terminal emulator for the X 
ii  rxvt-unicode [x-term 9.02-1              RXVT-like terminal emulator with U
ii  terminal.app [x-term 0.9.4+cvs20051125-3 a Terminal Emulator for GNUstep
ii  twm [x-window-manage 1:1.0.3-3           Tab window manager
ii  wmaker [x-window-man 0.92.0-7            NeXTSTEP-like window manager for X
ii  xterm [x-terminal-em 232-1               X terminal emulator

Versions of packages gdm recommends:
ii  dialog             1.1-20071028-3        Displays user-friendly dialog boxe
ii  gdm-themes         0.5.1                 Themes for the GNOME Display Manag
ii  whiptail           0.52.2-11.1           Displays user-friendly dialog boxe
ii  xnest              2:1.4.1~git20080131-1 Nested X server
ii  xserver-xephyr     2:1.4.1~git20080131-1 nested X server
ii  xserver-xorg       1:7.3+10              the X.Org X server
ii  zenity             2.20.1-1              Display graphical dialog boxes fro

-- debconf information:
  gdm/daemon_name: /usr/bin/gdm
* shared/default-x-display-manager: gdm
"""

I can confirm that this still applies to GDM 2.20.5