The default installation of Python on Mac OS X Snow Leopard is version 2.6.1. According to the man page for Python on Snow Leopard, Python 2.6 executes as a 64-bit application by default.
If, for some reason, you need to run it as a 32-bit application, this can be changed at the command line:
The preference can be set in either the User or Local ﬁlesystem domain in Mac OS X, following the normal precedence rules. To unset it, presumably you would change the boolean to “no” — or perhaps even delete the “Prefer-32-Bit” key.
There is also an environment variable that can override this preference.