If you are running Gentoo for a few months/years, your /etc/portage/package.keywords will get quite messy -- whenever you've needed a package that was at the time not keyworded stable, you have added it to that file. Now each emerge -Davut world becomes a PITA because of dependencies for all those keyworded packages.
Solution: Generate a list of all necessary keywords, and use this list: cd /etc/portage
mv package.keywords package.keywords.bak
equery -i -N list | grep \~ | sed 's/.* \(.*\) (.*/<=\1/' \
You then might want to edit the newly generated package.keywords to list the packages you really want from testing, like dev-db/phpmyadmin or net-analyzer/nmap