what I use are 2 options.
Option 1: if theres a windows machine that you can throw 2003 or 2008 server on you can use like-wise open on the ubuntu machins and use domain users. Like-wise will even auto create home directories. then you can couple this with pam auto mount to auto mout a cifs share from the windows machine on login and unmount on logoff.
Option 2: if no windows machine you can also have pam auth users via a mysql database backend. Simply add users to the mysql database and they will exist on all machines then use nfs automount to mount home directories. This option will even preserve permissions accoss machines easily as the nfs server can pull auth from same database table. for this you would want a dedicated mysql server with highest security and a "need-to-access" policy. I use iptables, fail2ban and strong passwords for this.
Both are fairly easy to setup and both work well.