The concept of Public Domain is actually pretty difficult to do with software. It is kind of hard to "abandon" the fact that you wrote the software which means you will always be liable for it. And you can't attach a disclaimer of warranty because that obviates public domain :-P Throw in that France won't let you disown your copyrighted works, and the whole area about PD software grows real murky. Daniel Berlin could probably be more specific, I just know the rough bits here. This question has also come up on our firstname.lastname@example.org mailing list; there should be more details there.
In any case, if you want to provide users with lots of rights, then I'd simply recommend going with the Apache License.