Direct Link and Mirrored Here, from March 25, 2011:
The key point to remember is that in order to ensure that an operation / implementation is RESTful, these methods must be used as they were defined in RFC 2616, the HTTP 1.1 specification. If implementations abuse these methods, they not only depart from RESTful behavior, but also jeopardize the application’s ability to interoperate with other RESTful capabilities.