I have built a copy of the new and improved open-source WebKit on OpenDarwin, and with some work, I have built an application launcher that runs Safari with this WebKit. Web developers who do not wish to build the WebKit from source, but want to test content against this version, can use this as easily as they might use any other Macintosh GUI application.
Just download the disk image and follow the instructions in the Read Me. The md5 checksum is 7367c911425d888b81cb66549bf62eb2. I have tested this distribution on Tiger, but I believe it should work on OS X 10.3.9. This launcher does not replace the default system WebKit, and you can run "standard" Safari at any time -- even at the same time -- by launching Safari the usual way.
The launcher is a simple perl script, adapted from the run-safari script that's included with the WebKit open-source package. It tells the system to use the local, new copy of WebKit when it runs Safari. The popular plug-in PithHelmet does not work with this application wrapper, but you can use the standard Safari with PithHelmet just fine.