I have experienced this issue after upgrading from Ubuntu precise (12.04) to quantal (12.10). I believe something went wrong during the upgrade of libxml2, and the old version remained.
The way you can solve it is by downloading a newer build of libxml2 from the Quantal's lauchpad repositoring and installing it manually. Choose a build for your system, and install using:
sudo dpkg -i libxml2_2.8.0+dfsg1-5ubuntu2.1_amd64.deb
I couldn't solve it using apt-get/aptitude, as they seemed to detected the precise version as the latest.