I'm not sure this is a gnome-session issue but here's the description of my problem:

I click on Desktop -> Log Out Koto -> Log Out. After that gnome freezes for me. this happend now 3 times over two days. The can not run any programs anymore. I can only open the Applications menu using Alt+F1. 

here's what my .xsession-errors file is saying about this:

 id: 117f000001000113007931400000087070014
  handle: 6
  connection: 0x8130f18
  properties:
    ((name "DiscardCommand") (type "LISTofARRAY8") (value "rm") (value "-f") (value "/home/koto/.metacity/sessions/1141810059-7479-2002293980.ms"))
    ((name "CloneCommand") (type "LISTofARRAY8") (value "metacity"))
    ((name "RestartCommand") (type "LISTofARRAY8") (value "metacity") (value "--sm-save-file") (value "1141810059-7479-2002293980.ms"))
    ((name "Program") (type "ARRAY8") (value "metacity"))
    ((name "UserID") (type "ARRAY8") (value "koto"))
    ((name "RestartStyleHint") (type "CARD8") (value "^B"))
    ((name "ProcessID") (type "ARRAY8") (value "16576"))
    ((name "CurrentDirectory") (type "ARRAY8") (value "/home/koto"))
    ((name "_GSM_Priority") (type "CARD8") (value "^T"))
  id: 11c0a81e05000109790793100000257270003
  handle: 5
  connection: 0x8132c78
  properties:
    ((name "CurrentDirectory") (type "ARRAY8") (value "/home/koto^@"))
    ((name "ProcessID") (type "ARRAY8") (value "16578^@"))
    ((name "Program") (type "ARRAY8") (value "gnome-panel^@"))
    ((name "CloneCommand") (type "LISTofARRAY8") (value "gnome-panel") (value "--sm-config-prefix") (value "/gnome-panel-cQQZJa/"))
    ((name "RestartCommand") (type "LISTofARRAY8") (value "gnome-panel") (value "--sm-config-prefix") (value "/gnome-panel-cQQZJa/") (value "--sm-client-id") (value "11c0a81e05000109790793100000257270003") (value "--screen") (value "0"))
    ((name "UserID") (type "ARRAY8") (value "koto^@"))
    ((name "RestartStyleHint") (type "CARD8") (value "^B"))
    ((name "_GSM_Priority") (type "CARD8") (value "("))
  id: 117f000001000114180810100000074390003
  handle: 3
  connection: 0x80c9c00
  properties:
    ((name "CurrentDirectory") (type "ARRAY8") (value "/^@"))
    ((name "ProcessID") (type "ARRAY8") (value "16596^@"))
    ((name "Program") (type "ARRAY8") (value "evolution-alarm-notify^@"))
    ((name "CloneCommand") (type "LISTofARRAY8") (value "evolution-alarm-notify") (value "--sm-config-prefix") (value "/evolution-alarm-notify-jXW8TD/"))
    ((name "RestartCommand") (type "LISTofARRAY8") (value "evolution-alarm-notify") (value "--sm-config-prefix") (value "/evolution-alarm-notify-jXW8TD/") (value "--sm-client-id") (value "117f000001000114180810100000074390003") (value "--screen") (value "0"))
   ((name "UserID") (type "ARRAY8") (value "koto^@"))
    ((name "RestartStyleHint") (type "CARD8") (value "^@"))
  id: 117f000001000114149687500000091210004
  handle: 4
  connection: 0x80bc758
  properties:
    ((name "CurrentDirectory") (type "ARRAY8") (value "/home/koto^@"))
    ((name "ProcessID") (type "ARRAY8") (value "16611^@"))
    ((name "Program") (type "ARRAY8") (value "mail-notification^@"))
    ((name "CloneCommand") (type "LISTofARRAY8") (value "mail-notification") (value "--sm-config-prefix") (value "/mail-notification-mtwCWB/"))
    ((name "RestartCommand") (type "LISTofARRAY8") (value "mail-notification") (value "--sm-config-prefix") (value "/mail-notification-mtwCWB/") (value "--sm-client-id") (value "117f000001000114149687500000091210004") (value "--screen") (value "0"))
    ((name "UserID") (type "ARRAY8") (value "koto^@"))
  id: 117f000001000111211989600000096460005
  handle: 1
  connection: 0x80c9e20
  properties:
    ((name "Program") (type "ARRAY8") (value "gkrellm2"))
    ((name "UserID") (type "ARRAY8") (value "koto"))
    ((name "RestartCommand") (type "LISTofARRAY8") (value "gkrellm2") (value "--sm-client-id") (value "117f000001000111211989600000096460005"))
    ((name "CloneCommand") (type "LISTofARRAY8") (value "gkrellm2"))
    ((name "ProcessID") (type "ARRAY8") (value "16580"))
  id: 117f000001000114181007000000165360000
  handle: 15
  connection: 0x8140300
  properties:
    ((name "CurrentDirectory") (type "ARRAY8") (value "/home/koto^@"))
    ((name "ProcessID") (type "ARRAY8") (value "16607^@"))
    ((name "Program") (type "ARRAY8") (value "deskbar-applet^@"))
    ((name "CloneCommand") (type "LISTofARRAY8") (value "deskbar-applet"))
    ((name "RestartCommand") (type "LISTofARRAY8") (value "deskbar-applet") (value "-Window manager warning: Lost connection to the display ':0.0';
most likely the X server was shut down or you killed/destroyed
the window manager.
The application 'gkrellm2' lost its connection to the display :0.0;
most likely the X server was shut down or you killed/destroyed
the application.
The application 'gnome-session' lost its connection to the display :0.0;
most likely the X server was shut down or you killed/destroyed
the application.
-sm-client-id") (value "117f000001000114181007000000165360000") (value "--screen") (value "0"))
    ((name "UserID") (type "ARRAY8") (value "koto^@"))
  id: 117f000001000114181009500000165360002
  handle: 37
  connection: 0x80c9d78
  properties:
    ((name "CurrentDirectory") (type "ARRAY8") (value "/home/koto^@"))
    ((name "ProcessID") (type "ARRAY8") (value "26697^@"))
    ((name "Program") (type "ARRAY8") (value "nautilus^@"))
    ((name "CloneCommand") (type "LISTofARRAY8") (value "nautilus"))
    ((name "RestartCommand") (type "LISTofARRAY8") (value "nautilus") (value "--sm-client-id") (value "117f000001000114181009500000165360002") (value "--screen") (value "0"))
    ((name "UserID") (type "ARRAY8") (value "koto^@"))
    ((name "_GSM_Priority") (type "CARD8") (value "("))



the only thing I can do is to switch to the console and restart gdm.  
my big problem is that I can't reproduce the bug. It seems to happen from time to time.