6.4.7 Mixed stable/testing/unstable system

apt-show-versions can list available package versions by distribution.

     $ apt-show-versions | fgrep /testing | wc
     ... how many packages you have from testing
     $ apt-show-versions -u
     ... list of upgradeable packages
     $ aptitude install `apt-show-versions -u -b | fgrep /unstable`
     ... upgrade all unstable packages to their newest versions