sys-kernel/vanilla-sources-2.6.28_rc{1,2} is lacking an amd64 keyword. The same thing happened with pre-release 2.6.27 kernels. At that moment the 'reason' for the lack of keywording was broken nvidia drivers, but that's not an kernel issue, is it? I'd like to see this bug fixed instead having to file it for every new release cycle.
Then please stop filing bugs for missing keywords every new release cycle. It's standard procedure to drop keywords for all arches and then have the arch teams add them back in due time. If you don't want to see that process, then simply add 'sys-kernel/vanilla-sources **' to /etc/portage/package.keywords. If that doesn't fix your issue with our policy, then please reopen this bug and argue why policy needs to change.
Off the top of my head, you could perhaps argue that with each version release, a bug is opened that amd64 users can vote on to get a specific version keyworded.