It's implicitly in the apt-get manpage, but I didn't see it until now - downgrading Ubuntu/Debian packages (for example to remove packages from an Ubuntu-PPA or a manually installed .deb with a newer version): Just add /distribution-name to the package when reinstalling.
For example (Ubuntu):
apt-get install --reinstall libgnutls26/oneiric libgcrypt11/oneiric
For example (Ubuntu):
apt-get install --reinstall libgnutls26/oneiric libgcrypt11/oneiric