OS X 10.5.8 came with Python 2.5, and had PyObjC already installed.
I installed Python 2.6 from the python.org site, and PyObjC isn't there.
I can't find a download to install PyObjC on my Python 2.6 install. Is checking out the PyObjC trunk and trying to build it my only choice? Will that work "out of the box"?
If your goal is to write software that will work on other people's computers, you shouldn't touch the default Python installation. If you simply cannot live without 2.6, then you're responsible for re-creating everything on your own, and that's not going to be a point-and-click process by any means.
Apple includes PyObjC with their Pythons that come with OS X 10.5 and 10.6. It is not part of the python.org installers. But it should be easy enough to install. Just install setuptools to the python.org python 2.6 following the instructions here. Then use easy_install-2.6 (which will have been installed in /Library/Frameworks/Python.framework/Versions/2.6/bin and may already be on your $PATH) to do:
easy_install-2.6 pyobjc==2.2b2
as described here. If you want to live on the bleeding edge, you could try installing directly from the svn repository as there has been a lot of work recently, primarily in support of 10.6.
If that seems like too much work, you could install an older version and all dependencies including python via MacPorts:
sudo port install py26-pyobjc2
You should probably try to build PyObjC from trunk, which will work fine on the official Python 2.6 distribution, but not on Python 2.5. There are quite a lot of fixes in the trunk right now that weren't in 2.2b2, which afaik. is the most current version available through easy_install.
There are some little snags that you may run into when building with py2app on 10.5 + 2.6 + PyObjC 2.2 (which for a lot of reasons is what you should probably do, instead of using the Xcode templates from 10.5 that build differently), especially if you still have Python 2.5 installed somewhere, so you'll probably want to build and install py2app from trunk as well, this particular issue I ran into with PyObjC 2.2 on 2.6 on 10.5 has been fixed by now.