Snow Leopard comes with its own Python and Numpy installation, located in /usr/bin, but in order to install Scipy it is necessary to install a “generic” version of Python.
Run the Python 2.6.4 installer.
Run the NumPy 1.3.0 installer.
Run the SciPy 0.7.1 installer.
It will be installed in /Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6
This information was taken from here.
Raul A. Monsalve 