Content (under whatever license) is 'enclosed' when it is contained behind a barrier such as proprietary encryption, a digital lock or a paywall. Enclosure does not restrict the content itself, but restricts access to the content; access is granted (typically under some other name) only via some concession, such as payment, or provision of personal information.
To my understanding, all of Flat World's content will now be enclosed behind a paywall. OERu assessments enclose assessment content. This mailing list (OER-community) encloses content behind a subscription requirement (I can't even link to discussions in my newsletter; all non-subscribers see is a barrier).
Enclosure is an important concept because it leads to 'conversion'. The process of conversion is one where what was once a resource that could be freely accessed is (for all practical purposes) accessible only through a barrier of some sort; in other words, the content is free, but has been effectively completely enclosed. This is what happened (for example) to many UseNet newsgroups. It almost happened to Wikipedia, and would have happened, has Google not intervened.
It seems to me that this is only a problem insofar as the cost of making and publishing a copy of "enclosed" but openly licensed material outweighs the value of doing so for each person in the world. It must be a problem on both ends.
There is a lot with format, etc., you can do to make copying a pain in the ass, and I don't think you are required to provide access to the "source code" in the same way you are with software, despite the fact that an educational resource may use a lot of software.
But I also suspect that OER's just aren't seen as that valuable. Is there an open educational resource as important as, say, BASH? Not that I'm aware of.
Also, the possibility of commercially re-distributing free content would be one of the main incentives to un-enclosing it. If Pearson is charging you $10 to access an OER, maybe I should copy it and offer it for $1. That might not be exactly what Stephen has in mind, but its a step.
I suspect a big part of the problem is just cultural at this point. It is well established that I can take a Red Hat Linux CD, change the name and try to sell it to you, or just give it away. I wouldn't win a lot of praise for that, but it is accepted. Would the same apply in the OER world if I was just copying openly licensed resources from behind paywalls?
I would note that it would be pretty easy for, say, Gates or Hewlett, to fund a project to just copy all OER's from behind paywalls and publish them.