Open Source Ecology talk:General disclaimer
To place a work in the Public Domain means you have relinquished Copyright.
But (perplexingly) Copyright must acutally be retained in order to apply a Copyleft License.
A Public Domain work can be 'captured' or 'enclosed' by other Copyright holders in that they can build upon that work (making changes) and then release it as a Proprietary product - withholding the virtual sources (design) from the end users. The BSD licenses, Apache, Artistic and others are similar in allowing such enclosure. Microsoft and Apple have used alot of Public Domain and *BSD code in this way.
The primary purpose of the GNU GPL (the most mature and popular of all Copyleft licenses) is to defend against such unsocial behaviour, but as with any Copyright license, it can only be applied by a Copyright holder that does not relenquish Copyright.
Your friend, Patrick Anderson -- AGNUcius 17:50, 25 February 2008 (PST)