Here are some relevant log entries (daemon.log) in several failed boot attempts:

Feb 24 11:51:23 dragon init: plymouth-stop pre-start process (2448) terminated with status 1
Feb 24 11:51:25 dragon init: ssh main process (842) terminated with status 255
Feb 24 11:51:30 dragon kdm[861]: X server startup timeout, terminating
Feb 24 11:51:30 dragon kdm[861]: X server died during startup
Feb 24 11:51:30 dragon kdm[861]: Failed to start X server. Starting failsafe X server.
...
Feb 24 11:52:41 dragon init: plymouth main process (288) killed by SEGV signal
Feb 24 11:52:41 dragon init: apport pre-start process (895) terminated with status 1
...
Feb 24 11:52:49 dragon init: plymouth-stop pre-start process (2408) terminated with status 1
Feb 24 11:52:51 dragon init: ssh main process (775) terminated with status 255
Feb 24 11:52:56 dragon kdm[805]: X server startup timeout, terminating
Feb 24 11:52:56 dragon kdm[805]: X server died during startup
Feb 24 11:52:56 dragon kdm[805]: Failed to start X server. Starting failsafe X server.

The Xorg.?.log contains nothing interest. Xorg.failsafe.log contains something interesting:
X.Org X Server 1.7.6
Release Date: 2010-03-17
X Protocol Version 11, Revision 0
Build Operating System: Linux 2.6.24-25-server i686 Ubuntu
Current Operating System: Linux dragon.ttdev 2.6.32-28-generic #55-Ubuntu SMP Mon Jan 10 21:21:01 UTC 2011 i686
Kernel command line: root=UUID=f5c42ce3-68c4-42ff-ab82-4cd38b4008d0 ro quiet splash 
Build Date: 10 December 2010  05:53:04PM
xorg-server 2:1.7.6-2ubuntu7.5 (For technical support please see http://www.ubuntu.com/support) 
Current version of pixman: 0.16.4
        Before reporting problems, check http://wiki.x.org
        to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
        (++) from command line, (!!) notice, (II) informational,
        (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(++) Log file: "/var/log/Xorg.failsafe.log", Time: Thu Feb 24 11:52:56 2011
(++) Using config file: "/etc/X11/xorg.conf.failsafe"
(==) Using config directory: "/usr/lib/X11/xorg.conf.d"
(==) No Layout section.  Using the first Screen section.
(**) |-->Screen "Default Screen" (0)
(**) |   |-->Monitor "Configured Monitor"
(**) |   |-->Device "Configured Video Device"
(==) Automatically adding devices
(==) Automatically enabling devices
(WW) The directory "/usr/share/fonts/X11/cyrillic" does not exist.
        Entry deleted from font path.
(==) FontPath set to:
        /usr/share/fonts/X11/misc,
        /usr/share/fonts/X11/100dpi/:unscaled,
        /usr/share/fonts/X11/75dpi/:unscaled,
        /usr/share/fonts/X11/Type1,
        /usr/share/fonts/X11/100dpi,
        /usr/share/fonts/X11/75dpi,
        /var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType,
        built-ins
(==) ModulePath set to "/usr/lib/xorg/extra-modules,/usr/lib/xorg/modules"
(II) The server relies on udev to provide the list of input devices.
        If no devices become available, reconfigure udev or disable AutoAddDevices.
(II) Loader magic: 0x81f0e80
(II) Module ABI versions:
        X.Org ANSI C Emulation: 0.4
        X.Org Video Driver: 6.0
        X.Org XInput driver : 7.0
        X.Org Server Extension : 2.0
(--) using VT number 8


Fatal server error:
xf86OpenConsole: VT_WAITACTIVE failed: Interrupted system call
lease consult the The X.Org Foundation support 
         at http://wiki.x.org
 for help. 
Please also check the log file at "/var/log/Xorg.failsafe.log" for additional information.

 ddxSigGiveUp: Closing log