The big bummer of Common Access Cards (CAC) is that there hasnt been a real good solution to get it working in Linux. I've seen it done for OS X but not for Linux. Well the guys over at HR Geeks did a writeup on how to get it working, worth a look if you have made the jump to being full-time Linux.
http://www.hrgeeks.com/2008/11/21/using-a-dod-cac-with-ubuntu-and-firefox/
http://www.hrgeeks.com/2008/11/21/using-a-dod-cac-with-ubuntu-and-firefox/