Single sign on usually refers to the ability to use a single username and password to log into any machine on your network. Ideally, this is managed by a central login database application. Logging into a Windows domain is a good example of this. For example in Windows 2000, the Active Directory suite handles all the login & permissions overhead.
Centralized Linux logins are generally handled by "NIS".
Centralized Windows logins on a Windows domain, in which the domain controller is a Linux and not a Windows server, is handled by the Linux Samba application.
Both seem to be covered on the site you mentioned.
I believe LDAP is a database system designed primarily to store user data.
Many centralized access, authentication & accounting schemes such as TACACS & RADIUS, and I think active directory too, can / do use LDAP databases to store login and permissions information about users logging into servers.
I'm not sure if NIS and Samba use or can use LDAP / MySQL databases for this type of activity.
I was jsut stumbling along and found this site .. and humourously I am currently implementing a "SingleSignOn" structure for my linux servers.
My preference was to use mysql, and while there isnt a pam module to access it, there is an nss library for it(well actually there are two). The one installed on our systems is libnss-mysql which can be found at http://libnss-mysql.sourceforge.net/. It works rather well, and it comes with both rpm and tarball download options. One of the other techs here did the install, I'm supposed to play with it later next week, but it seems to be great... What it basically does is make the mysqld literally act like your /etc/passwd file, so that other apps can treat the virtual users as if they were real users without that app needing to support mysql.