As with any copyright licence, software developers who use any
version of the GPL can also grant additional permissions to
recipients for code that they hold the copyright of. That is, they
can say that you can distribute the software under the terms of the
GPL, and they can additionally say that, at your option, you can
also distribute the software in this way or that way.
About such additional permissions, the following words are proposed
for GPLv3, in discussion draft 2: "When you convey a copy of
a covered work, you may at your option remove any additional
permissions from that copy, or from any part of it." As I
see it, these words actually don't change the nature of such
additional permissions at all. This topic has come up a few times
when I've been discussing GPLv3 with people, so here's my
understanding of this issue.