W3C:

2005-06-20: The Voice Browser Working Group has released Authorizing Read Access to XML Content Using the Processing Instruction 1.0 as a Working Group Note for information only. This note describes a mechanism being used in the industry that allows a content provider to use a processing instruction embedded within XML content to specify the access policy of that content. Implementors should perform their own security analysis. Visit the Voice Browser home page. (News archive)