You've probably heard the announcement: Martin Pitt's PPA is being deprecated. This is in an effort to push folks towards the new official apt.postgresql.org.
See the announcement here: http://www.postgresql.org/about/news/1432/
Also, see the deprecation notice on Martin's PPA:
Since we have quite a few customers using Martin's PPA for PostgreSQL packages under Ubuntu, I wrote a little shell script to switch repositories if you're using Martin's PPA and just install the apt.postgresql.org repository if you are not.
You can find it here: https://github.com/pgexperts/add-pgdg-apt-repo