New genkernel, and I found that my kernel, System.map and initramfs were no longer in the same order as before, causing problems with my grub configuration. I considered rebuilding some old kernels (just to have a fallback), or just getting rid of the old kernels... and then I decided to rename all the old kernels to match the new genkernel format:
cd /boot
for file in /boot/*genkernel*
do
newfile=$(echo $file | sed -e 's/genkernel-x86_64-//g' -e 's/-gentoo-sm/-gentoo-x86_64/g' -e 's@^/boot/kernel-@/boot/vmlinuz-@g' -e '/initramfs-/s/$/.img/g')
mv $file $newfile
done
grub-mkconfig -o /boot/grub/grub.cfg
I guess this looks like a script but it was really just a single command that I cut and pasted to the various gentoo boxes to "fix" them.
This will be important for a very brief time, as I don't hold onto kernels for very long.