Here's another hopefully useful how-to for getting Thinkpad T420 fingerprint reader running under Ubuntu 11.10:

Install required packages:

sudo apt-get install fprint-demo libpam-fprint

Add udev rule for setting the group ownership of fingerprint reader device:

echo "ATTRS{idVendor}==\"147e\", ATTRS{idProduct}==\"2016\", MODE=\"0664\", GROUP=\"fingerprint\"" \
    | sudo tee /etc/udev/rules.d/99-fingerprint.rules

Create group and add user to that group:

sudo groupadd fingerprint
sudo gpasswd -a lauri fingerprint

Reconfigure PAM and enable "Fingerprint reader":

pam-auth-update

Reboot the machine so the udev would create the fingerprint reader device node with proper permissions. At the moment I am not familiar how to force udev to recreate the node and unplugging the fingerprint reader is not "feasible".

Finally scan your fingerprints:

fprint_demo

Both, authentication in the login manager and sudo, should work with fingerprints now!