Once something's in the GPL it's free software. It can't be taken back. Future releases under the same license can be halted, however.
I am not sure what you mean with `halted'; you can't take back a license like the GPL or any other free software license. And those copyright holders may not contribute new open source material to the new non-GPL version / release of Linux, but there's money to be made for them staying by with Linux even after a shift to a non-GPL form (and probably especially after a shift to non-GPL actually). As long as they stay with free software then that is good. By the way, `open source' is a different movement than free software; they might share our licenses but not our goals. See http://www.gnu.org/philosophy/open-source-misses-the-point.html for more information.
