I got a newsletter some time ago informing me that www.Yubico.com (or rather one of it’s partners) had added AD integration to the Yubikey, this should allow you to use the Yubikey to login to windows something that could be quite interesting..
Read more here;
http://www.yubico.com/news/100204/
ps.
I also seem to recall stumbling across some PAM and RADIUS implementation, so you might want to check their forum if you are into Yubikeys.
http://forum.yubico.com/