Usually this setting could be placed in the hadoop-site.xml file used by all daemons on all nodes. In an environment such as Amazon EC2, though, where a node is known by multiple addresses (one public IP and one private IP), it is preferable to have the SecondaryNameNode connect to the NameNode over the private (unmetered bandwidth) IP address, while you connect to the public IP address for status pages. Specifyingdfs.http.address as anything other than 0.0.0.0 on the NameNode will cause it to bind to only one address instead of all available ones.
In conclusion, larger deployments of HDFS will require a remote SecondaryNameNode, but doing so requires a subtle configuration tweak, to ensure that the SecondaryNameNode can communicate back to the remote NameNode.