OK, now that's interesting. When paulj says <i>"RedHat maintainers literally did check in *patches* to CVS, as the source of their kernel src.rpm"</i>, is that something that used to be the case, but isn't any more, or has it never been the case?
Fundamentally it seems to me that the purpose of the 'preferred form' language is to stop distributors of GPLed code from distributing in a form designed to make life harder than it need be for downstream consumers. In this specific instance people from Red Hat management have been making statements that this change was specifically intended to do just that, as a means of competing with Oracle, Novell etc.
If the reason for the change is that the canonical source used to be a pristine Linus kernel and a bunch of patches, and now it isn't because you've moved to git, then there wouldn't be any GPL concerns. But if that's all that's happened then it might have helped if someone had just said so, rather than bringing competition into it.