"Few if any legal systems have a process for reliably donating works to the public domain. They may even prohibit any attempt by copyright owners to surrender rights automatically conferred by law, particularly moral rights. An alternative is for copyright holders to issue a licence which irrevocably grants as many rights as possible to the general public, e.g., the CC0 licence from Creative Commons." (Source: http://en.wikipedia.org/wiki/Public_domain#Dedicating_works_...)
I suppose the better question is whether someone can rescind their license for their work and prevent future legal use of that software. I'd like to think that's not possible either, but I have no clue to be honest.