After a recent upgrade to Mountain Lion I ran into a few small problems with gcc and some python modules. I ended up having to reinstall pip and virtualenv, but I couldn't get PIL to compile correctly. After some searching..
I followed the most voted instructions from Here.
Additionally, apple moved the location of the gcc tool, so you'll need to create a symlink:
sudo ln -s /usr/bin/gcc /usr/bin/gcc-4.2