ia32-libs and ia32-libs-gtk were used by a lot of people to run 32-bit software on an amd64 installation. They have been removed recently from the archive (bug #535645), on the basis they are hackish packages (which is true), and replaced by something even more hackish, ia32-apt-get.

ia32-apt-get failed to work correctly for a lot of persons, which broke a lot of systems. Fortunately Mark Hymers reintroduced the packages in the archive. In the long term, those packages will be replaced by multiarch support (planned for squeeze), which will allow, among other things, the installation of 32-bit packages on an amd64 installation.