No. Copyright is implicit. Something can only be in the Public Domain if it's explicitly placed there (and supposedly, not all countries even recognize Public Domain, hence the creation of the WTFPL[1]).
> and supposedly, not all countries even recognize Public Domain
I'm not sure the phrasing is correct, at least for the most common example of this issue (european copyright tradition of moral rights): many european countries split the anglo-saxon copyright into estate rights (which are economic) and moral rights which are generally perpetual, inalienable, and imprescriptible.
As a result, while an author can assign (or waive) their estate rights they can not assign or waive their moral rights.
Unless there are additional special cases in the country's lawbooks an author in a moral-rights country can not put his works in the public domain (since it would require waiving rights he can't waive). Either the works can not be copyrighted (and are intrinsically in the public domain) or they will fall into the public domain when the author's rights expire.
It's common sense - in most european countries you can allow others to earn money from your work, modify it and distribute etc, but you can't allow others to lie that they made something, not you, when the truth is that you made it.
So it's not public domain, because some rights are forever yours, but you can very well allow anybody to use modify and distribute your creation.
Actually, something can only be in the Public Domain if it it's not covered by copyright, or if its copyright has expired; saying "I put this in the public domain" might have no effect whatsoever.
I don't know of any countries that don't recognise public domain (that is, implement eternal copyright).
The big example usually given is France. Under French law there are several "moral rights" like anthologizing. Here's the best way I can express this: when _why left the Ruby community and deleted his online presence, several people created collections of all of the open-source software which _why had written, without _why's permission. That sort of collection is illegal under French law; an English translation of the relevant statute says, "The author alone shall have the right to make a collection of his articles and speeches and to publish them or to authorize their publication in such form." That right is not transferable, you can't get rid of it, etc.
There is a related nasty moral right in France about withdrawing your copyrighted works -- "Notwithstanding assignment of his right of exploitation, the author shall enjoy a right to reconsider or of withdrawal, even after publication of his work, with respect to the assignee." If I am reading the other terms on this page right, that might not apply to software in particular, but it could probably apply to Creative Commons licensed writing, e.g. Wikipedia.
Even worse, French law does not permit a "mostly complete" copyright contract, as I understand the legal history -- so the French courts have actually said things like, "this contract tries to give away your moral rights, that's legally impossible, so the whole license is legal nonsense, so there never was a license, so it is totally proprietary."
You could release it under a copyright licence that is de facto the same as the public domain. It's "copyrighted to you", but anyone can do anything on it.
No. copyright is automatically applied when the work is created. You have to explicitly give more relaxed rights, aka licenses and copylefts, for people to know whether they can use it.
