For our DAD/miniDAD project a few years ago, I came up with a solution of how to configure Exim to use a smarthost if configured in LDAP. There wasn't anything really revolutionary about that, but I found it quite nifty. So much, that I expanded that a bit for my Exim and LDAP presentation recently, and even showed how to create a specific schema for storing similar values.
The true beauty of having an application (in this case the Exim MTA) pull its configuration out of a directory shows itself when you then actually use it.
Reconfiguring a dozen servers on the fly (ok, not a full dozen, but
close) was a cinch. One
ldapmodify later, they all sent their mail somewhere
else. (In a controlled manner. :-) )