A short quote from WikiPedia might help to clear up the apparent confusion about XML namespaces, what they are and how they are (actually) used:
A namespace name is a uniform resource identifier (URI). Typically, the URI chosen for the namespace of a given XML vocabulary describes a resource under the control of the author or organization defining the vocabulary, such as a URL for the author's Web server. However, the namespace specification does not require nor suggest that the namespace URI be used to retrieve information: it is simply treated by an XML parser as a string. [...] Although the term “namespace URI” is widespread, the W3C Recommendation refers to it as the “namespace name.”
Upon encountering a namespace reference, any good XML parser will require you to have declared its presence first. But, it, “the parser,” does not care what that reference is. “As long as the identifier has previously been made known to me, it is a valid parse.” Of course, the consumers of an XML document typically do care ... both what namespace-identifier you use, and what namespace-name you associate with that identifier.
