LightDM is the new display manager used by Ubuntu 11.10 Oneiric Ocelot that allows users to submit their account credentials to login to a session. The problem with this display manager is that at logon screen, the numpad is locked and you are obliged to enable it manually to be able to submit your password.
To solve this, open the terminal (Ctrl+Alt+T) and run this sequence of commands:
sudo apt-get install numlockx
echo "greeter-setup-script=/usr/bin/numlockx on" | sudo tee -a /etc/lightdm/lightdm.conf
Reboot now your system to check if it worked.