If openbox permanently tries to run after gnome session is started
--- /usr/share/applications/openbox.desktop.orig 2012-09-26 19:24:29.000000000 +0400
+++ /usr/share/applications/openbox.desktop 2012-09-26 19:21:21.000000000 +0400
@@ -5,3 +5,4 @@
Exec=openbox-session
Icon=
Type=Application
+X-GNOME-Provides=windowmanager