Those of us who have actually read through many of the Open Source licenses and have spent a fair bit of time mulling and discussing their consequences take a lot of things for granted.
One of those things is that once a program, or anything else, is released under an Open Source license you can't just take it back. Maybe this seems obvious to you, or maybe not, but it isn't to everyone. Perhaps especially as we depart the realm of software where most developers involved with Open Source have given at least passing thought to the implications of the GPL and other such licenses.
This was brought home to me the other week in this comment on Flickr by Lane Hartwell (username "fetching"). (The context isn't especially relevant to this discussion; I suggest reading the whole heated thread if you're really interested.) "[this discussion] has brought attention to some issues and may help change things on both ends. Who knew that CC Licenses were permanent? Flickr sure doesn't tell you when you choose that option."
There are a variety of of issues raised in this case, but the one I want to focus on is that a photographer initially posted a picture on Flickr under a Creative Commons license and subsequently changed its license to the default "All rights reserved" (i.e., any use beyond that allowed by Fair Use requires the explicit permission of the photographer). There is a family of Creative Commons licenses. They vary, essentially, in whether the licensed work can be altered and whether it can be used for commercial purposes. However, for our purposes here, we can just think of all of them as "Open Source licenses."
Physical world intuition might suggest that of course the copyright holder, the owner of the property in a sense, can unshare a work anytime he or she chooses. If I give you permission to borrow my car, I can certainly give you permission on a one-time basis or can withdraw that permission at any time (subject to any contractual agreements).
But Open Source licenses are different. Once I put a photograph, a novel, or a program out in the world under an Open Source license, it's out there. I can't go "never mind" and withdraw whatever rights the license granted in the first place.
I'm not saying that the copyright owner can't change the license. In the case of works to which multiple people have contributed, there are a variety of complications and legal theories around changing licenses, but that's a separate issue. The bits or the words or the arrangement of ink droplets that have already been released into the world remain covered by the Open Source license they were originally released under.
A Mattel court case involving their CyberPatrol software and a program by Eddy Jahnsson and Matthew Skala called cphack raised the issue of whether a GPL license could be withdrawn. However, the case was such that no definitive legal conclusion came about. In addition, there were questions over whether cphack was even properly licensed under the GPL.
In any case, the widespread opinion among those who work with Open Source licenses is that what's been released into the world can't be subsequently withdrawn. As stated in this FreeBSD document:
No license can guarantee future software availability. Although a copyright holder can traditionally change the terms of a copyright at anytime, the presumption in the BSD community is that such an attempt simply causes the source to fork.
In other words, if the license is changed to an "unfree" license, you don't get the right to enjoy any downstream changes--whether enhancements to a software program or touchups to a photograph. But the specific work that's been released to the world can't be withdrawn.