When would you put something in the DMZ

I have a client that is hosting their site and soon to host others. They use FTp and the Web server is multihomed (i know security risk 101, but I didn’t set it up) and communicates with a SQL server on the inside.
I have been going back and forthon this topic with a friend, the topic being when you would put a server in the DMZ and when you would map ports through the firewall.