I wanted to set up local accounts on my PowerPCBeigeG3 to allow passwordless login via GDM on Debian 3.1. Looking around I noticed I wasn't the only one looking.

This post on lists.ubuntu.com got me going in the right direction:

http://lists.ubuntu.com/archives/ubuntu-users/2005-April/032282.html

 [ /etc/pam.d/gdm ]
 #%PAM-1.0
 auth   requisite       pam_nologin.so
 auth   required        pam_env.so
 auth  sufficient      pam_listfile.so sense=allow file=/etc/passwordless item=user
 @include common-auth
 @include common-account
 session        required        pam_limits.so
 @include common-session
 @include common-password

 [ /etc/passwordless ]
 <username>

This worked for me. Except for the fact that the line is put into /etc/pam.d/gdm it isn't a GDM specific tweak. You should be able to do the same with any service that uses PAM. I'm only using it for GDM and have GDM configured to not allow remote logins.

You may also want to disable the 'lock' feature of your screen saver for the passwordless gdm login accounts.