There is a very interesting article about the GPL copyright license and the BSD copyright license, and this author’s view that the public domain is the only way to go. This is a very interesting take on both licenses.
His (her?) view is that both licenses place restrictions on the user (as he suggests all licenses do). However, I would beg to differ with the assessment on both licenses…
The GPL license does place restrictions on the user; however, those restrictions are there to preserve the freedom to change, modify, and give away the source code. That’s it. The restrictions are there to preserve freedom.
The BSD license places restrictions that basically say the user is responsible for the software, and says nothing about anything else. The BSD license was designed to preserve the freedom to do whatsoever you will with the software (including putting it into proprietary systems and not releasing source code).
However, the public domain basically places no restrictions whatsoever on your software. Thus, someone can appropriate the software, start selling it, claim they wrote it, and more – without any recourse for you, the original author. It is for this reason that the Public Domain is not where you want your software.