That's happening to me after clean install of dapper. Nothing useful in the logs but the .xsession-errors.
On the last line it was written "Executing default" instead of "Executing /usr/bin/gnome-session" -- gdm fails to create session https://launchpad.net/bugs/51233 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
