Nathan Myers thinks out loud about the revocable nature of open source licenses:
A quick scan through the Free Software licenses I have immediately on hand showed one thing in common: none say the rights are waived perpetually or irrevocably. On the face of it, it seems, I could release a program under the GPL, and then announce five years later that it and all derived works are under my private control again.
The rest of the article goes into ways to resolve this sticky issue. IANAL so most of this stuff flies over my head.