1. Grab source of Mod_python. Actually 3.3.1.
2. Unpack with your favourite tool
3. Change to source directory

4. Follow steps below

   $ ./configure --with-apxs=/usr/sbin/apxs
   $ make
   

   Compilation will fail with this error:

   ...
   connobject.c: In function ‘_conn_read’:
   connobject.c:142: error: request for member ‘next’ in something not a structure or union
   apxs:Error: Command failed with rc=65536
   ...
   

5. Download this patch for src/connobject.c

6. Save it to the same directory you saved mod_python (presumably the Downloads directory of OS X)

   $ cd ..
   $ patch -p0 < mod_python-3.3.1-apr.patch
   $ cd mod_python-3.3.1
   $ make
   $ sudo make install
   

Now the module resides in /usr/libexec/apache2. To finish the installation, you need to add mod_python to your httpd.conf:

$ sudo vi /etc/apache2/httpd.conf

and add

LoadModule python_module libexec/apache2/mod_python.so

After restarting apache you are ready to use python.

Updates:
2011-01-07: Rewrote the patch-part