The PyObjC repository is a subversion repository at http://svn.red-bean.com/pyobjc.
Martin Ott maintains an easy to install Subversion package for Mac OS X 10.3. It can be downloaded from his site.
You can fetch a snapshot of the current development tree using a WebDAV connection to http://svn.red-bean.com/pyobjc/trunk/, just copy the 'pyobjc' folder to a local disk.
PyObjC includes a large number of unittests. These can be started using a test command in setup.py: python setup.py test.
The best way too contribute to PyObjC is by sending patches or (links to) examples to the list. Soon enough someone will get bored with applying your changes and you'll get access to the repository :-).