> That's like having a clause in the GPL that says at any given time, the author can revoke the GPL and sell the copyright to Microsoft.

Which, unless the author has specifically assigned copyright to someone else, you can totally do. A similar situation arises with dual-licensed software. Of course, older versions of the software still 'live on' under the GPL, the author is implicitly making a fork.

This is one of the reasons the FSF wants you to assign it copyright to stuff: http://www.gnu.org/licenses/gpl-faq.html#AssignCopyright