If you are using the default (system) Python, I found that the only extra dependency you need to install is pkg-config. I used homebrew for this (http://mxcl.github.com/homebrew/ and “brew install pkg-config”). Alternatively, you can install it directly from the source obtained at http://www.freedesktop.org/wiki/Software/pkg-config. Be sure to get the 0.25 release at http://pkg-config.freedesktop.org/releases/pkg-config-0.25.tar.gz, as 0.26 has an unnecessary and expensive dependence on glib. To install, unpack and follow the instructions from the homebrew formula:
(mkdir -p /usr/local if needed)
./configure --disable-debug --with-pc-path=/usr/local/lib/pkgconfig:/usr/lib/pkgconfig:/usr/X11/lib/pkgconfig
Don’t worry about the libpng problem - it is fixed in matplotlib 1.1.0, which is available on PyPI. I can’t vouch for pip, but “sudo easy_install matplotlib” did it for me.
Regards and please let me know how it went!