How do I unlock a user account?

Try sudo usermod -U username to unlock your account.

I just discovered your questions after it affected me too. Please take a look at following bug report: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/851055 It seems somethin is broken in lightdm (new default display manager in Ubuntu 11.10). The problem had first occured during the beta release of Ubuntu Oneiric and seemed fixed, but users are still reporting problems. At the moment there isn't much you can do. Subscribe to the bug report and show that it affects you. Wait for a new fix or downgrade to an older lightdm version.

edit: Of course you can do something. Remove lightdm as display manager and install "good old" gdm display manager. Open a terminal and enter: sudo apt-get remove lightdm to remove lightdm, next enter sudo apt-get install gdm to install gdm. Reboot

Hope lightdm gets fixed soon!

Changing the default window manager to lightdm (as proposed here) solved that issue for me.