PyObjC adds support for Tiger specific technologies - however not in the current released build. You currently need to grab the source via subversion.
First, download subversion for Mac OS X from Martin Ott’s site. The 10.3 installer seems to work okay for 10.4.
Second, follow the instructions for downloading the PyObjC source:
svn co http://svn.red-bean.com/pyobjc/trunk/pyobjc/
Finally, follow the installation instructions. The short version is:
cd pyobjc
python setup.py bdist_mpkg --open
And then follow the instructions in the installer.