The GPL explicitly disallows revoking the license. It has occurred, however, that a company (Mattel) purchased a GPL copyright (cphack), revoked the entire copyright, went to court, and prevailed [2]. That is, they legally revoked the entire distribution and all derivative works based on the copyright. Whether this could happen with a larger and more dispersed distribution is an open question; there is also some confusion regarding whether the software was really under the GPL.