My Intellectual Property professor (hereafter "The Preacher") on the public domain:
"When can IP rights be reclaimed from the public domain? I envision it as a giant rift, where the rights drop from the heaven that is protection into the rift that is the public domain. When you lose protection, they drop into the public domain. Once they fall from heaven, they can only be prayed out. You must clutch your rosary beads and pray them out by changing the law, or by resurrecting abandoned trademarks."
Case in point: Mickey Mouse's copyright protection was set to expire, until Congress extended copyright protection laws. Mickey resurrected into heaven.