Update: new overlay for xscreensaver 5 (see below)
A Linux driver for the fingerprint reader found in some of the newer Thinkpads has been released (I have a T43p). I got it to work on Gentoo using the instructions and ebuilds provided by Tobias Sager, which are based on the original instructions found in the very useful Thinkwiki. For getting xscreensaver to work, I used the alternative patch found in linux.spiney.org. It works! Now I can login and unlock my screen using the fingerprint reader.
One thing that caused me trouble is that for non-root users to be able to authenticate, they have to be able to read the files under /etc/bioapi/pam, so make sure the “usb” group has read permissions there.
I have made a modified xscreensaver-4.24 ebuild (with version 4.24-r1) that you can put in your overlay and that includes the patch for alternative PAM authentication and the patch for GL screen selection. You can download it here.
Update: I have made a version of the patch for xscreensaver 5, and updated the overlay to include a modified xscreensaver-5.00 ebuild (with version 5.00-r1). Seems to work fine.