Security by Obscurity
Not effective when it:
-  is the only security
 -  is not real obscurity
 -  prevents accurate determination of a product's security
 -  is given irrational confidence
 
Valid when it:
-  helps to avoid vulnerability targetting (hiding equipment and versions)
 -  complements other measures
 -  hinders social engineering attacks (e.g. hide hostnames behind firewall)
 -  is used to protect other measures such as intrusion detection