Something must have went wrong with Apport (the crash report app). Some users are left unable to boot in Ubuntu 11.04. This is the quick fix:
1) When it stalls at the Ubuntu load screen press [CTRL][ALT][F2] and get to a terminal.
2) Sign in (username, then password - Must be a SUDOer)
3) Run the following command 'sudo gdm' (Enter your password again)
4) Enjoy your session.
An update should be available soon with the permanent fix, and now you'll have a GUI to do it in. Remember this is a 'duct tape' solution and you will need to update.
(Update: If you're really tired of bugs like this on end-user systems consider Hexxeh's Chromium OS builds, and if you're tired of problems like this on server systems then learn about how your system works or try Debian - you can take your MySQL databases with you.)
(Update 2: If the problem persists just remove apport after using this quick fix, I also removed the plymouth-theme-* packages because I just don't need the boot time confusion.)
(Update 3: After receiving more user input it seems some users are seeing the message erroneously. If you've been messing around with GDM and X11 settings. getting the old org.gnome.DisplayManager problem if you don't sudo? try this. "sudo nano /etc/X11/default-display-manager" Does it say /usr/bin/gdm and if so is your GDM in usr/bin or /usr/sbin - because all of you with it in sbin will need to add that missing S. Then press Ctrl-o to save and ctrl-x to exit nano. Reboot and enjoy.)