The ItemGuid property returns a string which uniquely
identifies the current news item. If this property is defined, it is
guaranteed to be a unique, persistent value. It is important to note
that this string does not have to be a standard GUID reference
number, it can be any unique string. In many cases it is the same
value as the hyperlink returned by the ItemLink property,
although an application should never depend on this behavior. If
there is no unique identifier associated with the current item, this
property will return an empty string.