During a discussion with Mark Pilgrim over some Atom extensibility issues, I ran into some questions which are likely to be controversial:

Who owns a XML namespace?

A XML namespace URI is supposed to be just an opaque string that is unique enough to identify a namespace.  It is not a place so the question of ownership is different the question of link ownership.  I suppose one could apply trademark or copyright laws to XML Namespace URIs.

If one can exercise legal control over XML namespaces, what are the legal implications of standard organizations like W3C owning key XML namespaces?  I am not a lawyer, no can I think like one so I'll punt this question.  But we are likely to see clauses appearing in specs related to XML namespaces.

Should XML namespaces be Open or Closed?

An open XML namespace allows third-parties to add new names according to a strict naming design pattern.  Are there use-cases where there is a need for XML namespaces to be open?  What about nested XML processor plugin frameworks that uses XML Namespace URI?

There is also the question of rogue tags being carried as a hidden beacon of sort across the Net and into Intranet.  If unknown tags are ignored by default everywhere, what are the security and legal implications?  If my web service logs all invokations and someone slips in copyrighted or illegal information into a SOAP call, am I liable?  There is a whole new type of cross-scripting issues in web services.