Berne Convention uses the term "public domain" as something that happens to works when copyright expires (absence of copyright), but it doesn't mean you can say an incantation like "i hereby release it to public domain" to make that happen. In many (most?) countries the incantation doesn't have a defined legal meaning as authors always have copyright to their works, the Berne Convention is implemented in national copyright law without talking about "public domain".

See here, Article 18 is the only place that mentions public domain: http://www.wipo.int/treaties/en/ip/berne/trtdocs_wo001.html#...