ThinkFinger is a driver for the SGS Thomson Microelectronics fingerprint reader found in most IBM/Lenovo ThinkPads.
Get if from here.
Use tf-tool to test ThinkFinger. You'll have to run this as root because a direct access to the usb devices is needed. Run tf-tool --acquire to generate a test.bir and tf-tool --verify to see if it identifies you correctly. tf-tool --add-user <username> acquires and stores your fingerprint in /etc/pam_thinkfinger/username.bir, which is needed for an authentification with pam.
PAM is the Pluggable Authentication Module, invented by Sun.
Change the file /etc/pam.d/other to look like this if you want to use your fingerprint to authenticate yourself on logon:
#%PAM-1.0 auth sufficient pam_thinkfinger.so auth required pam_unix.so use_first_pass nullok_secure account required pam_unix.so password required pam_unix.so session required pam_unix.so
Change this file to confirm the su command with a finger-swipe!
#%PAM-1.0 auth sufficient pam_thinkfinger.so auth required pam_unix.so use_first_pass nullok_secure account required pam_unix.so session required pam_unix.so
Don't forget to do a tf-tool --add-user root!