pam_ssh_agent is a pam module that spawns a ssh-agent and tries to decrypt
your private key using the password passed to previous modules in the pam
stack.

dependencies: 

 keychain: http://www.gentoo.org/proj/en/keychain.xml
 
add:

session    optional     pam_ssh_agent.so

to your /etc/pam.d/system-auth file or to every service you want to be
pam_ssh_agent enabled.
