In the US you can't put your code into the public domain. Every work you produce is copyrighted and while you can assign copyright to someone else, you can't "remove" it from a work. The only way for something to become public domain is for the copyright to expire (which is practically never at this point).
Not so - you just have to make an overt statement that you are abandoning your rights. For example, I hereby waive any and all copyrights in this Hacker News comment, now and forever. Enjoy!
See paragraph 22 in this entertaining and interesting opinion about the copyright status of user-created videogame levels for a more detailed explanation; as far as I'm aware nothing has changed since then: http://ftp.resource.org/courts.gov/c/F3/154/154.F3d.1107.htm...
I am not a lawyer, but I believe you are mistaken. In the US, so long as your are the legitimate holder of the copyright you may voluntarily waive all rights and surrender the work to the public domain. You may also user certain licenses such as those proposed by the Creative Commons which waive most rights without actually releasing it to the public domain.
Certain other countries are slightly different in that they recognize "Moral rights" that are perpetual and unwaivable.
