How can you run UNIX for ~150,000 users?

OldBen asks: "How do you handle email (or other services for that matter) for hundreds of thousands of users? Obviously, you can't just use a standard UNIX password file (or can you)?" Actually, you COULD use a password file...but that would be painful. There are ways to make unix use a database as a replacement for the password file. However I'm sure there are other ways of doing this. Are there any methods that would allow you to split user resources over a pool of boxes rather than just one huge monster box?

What say you all?