Taking my brain away from the election boondoggle this morning, after an emerge --sync, I was greeted with this:

!!! Your current profile is deprecated and not supported anymore.
!!! Please upgrade to the following profile if possible:
        default-linux/x86/2004.0
To upgrade do the following steps:
# emerge -n '>=sys-apps/portage-2.0.51'
# cd /etc/
# rm make.profile
# ln -s ../usr/portage/profiles/default-linux/x86/2004.0 make.profile

Searching the forums led to one post in german about this. Since I’ve got the luxury of more than one non-production test machine, I went ahead and followed the instructions. All seems to be fine.
A little more poking around in the gentoo documentation, and I found some pretty good advice on this upgrade, seems to be just a file system layout/naming convention change.