Securing a New Idea for the Public Domain?

piotru asks: "I live in Japan and am implementing a new idea in software. I was told that it deserves patenting, but I intend to make sure that it remains in public domain (preempting anyone else from patenting it). What can I do to make sure that no one else patents my idea? Here are some choices I have come up with: 1) Patent and apply free license; 2) Apply for the patent, then drop it (will be published with time stamp, and more than likely will be costly); or 3) Copyright the implementation (will have a time stamp, but will it remain compatible with the GPL?) Any ideas?" Some of these issues were touched on in an older story, but the basic premise remains a difficult issue. How would you go about releasing an idea to the public domain while making certain that close software couldn't co-opt it out from under you?