While I understand the thinking behind the desire for a mark indicating that information is in the public domain, I (along I suspect with others) would vehemently oppose the introduction of such a mark on principle.

Currently (in most of the world at least) all published information can be assumed to be in the public domain unless otherwise marked. That's not to say all information - the act of publishing is normally required (although not always necessarily by the owner or author of the information). The concept of copyright only applies to information you want to publish but retain ownership of.

Where copyright is claimed, it's a far far more acceptable approach to place the burden of maintaining it on the publisher than the reader. The introduction of a 'public domain' mark would reverse this entirely, requiring not only that an author decide to stop defending his or her copyright, but that that author must consciously decide to place their work in the public domain. Not good for the free flow of information.