Excellent posting from Anil John:
In the physical world, when an attacker is preparing to assassinate someone or bomb a target, the first thing that they will do is to determine how best to set up that attack. The phrase used to describe the initial phase of the set-up is called 'pre-operational surveillance'.
Unfortunately, the default configuration of most web services allow a potential attacker to do the digital equivalent of pre-operational surveillance very easily. In the digital world, these type of threats are often classified under the category of 'Information Disclosure Threats'. There are two in particular (there are more) that I would like to call attention to:
1. SOAP Fault Error Messages
2. WSDL Scanning/Foot-Printing/Enumeration