2008/8/6 Robert Rohde rarohde@gmail.com:
A related issue to the one Artur raises is that many legal systems have an absolute right to withdraw contractual offers if they have not already been mutually agreed upon. If you believe that a copyright license is a form of contract (which is probably true in at least some jurisdictions), then it is likely that the licensor can stop offering a copyleft license to new parties, even though they can't revoke it from any existing users.
There are two issues here.
The question of whether a copyright licence is revocable depends (at least in the common law jurisdictions, not sure whether civil law approaches this differently) on whether it is a bare licence or a contractual one. Bare licences can be revoked at will (unless there is an estoppel at play, for example) but contractual licences can only be revoked as provided for under the contract or under general principles of contract law.
On the second issue you're absolutely right, a person can simply stop making copies available under whatever licence, without affecting the rights of those who are already using a copy under the licence. It's rendered somewhat meaningless in the digital context, however.