To download a pre-built bundle containing the latest WebKit visit WebKit Nightly Builds.
To download the WebKit source tree:
Install a Subversion Client
Subversion (svn) is the source code management tool used by the WebKit Open Source Project. A Subversion 1.4 client for Mac OS X 10.4 is available.
Open Terminal
Terminal resides in /Applications/Utilities. The Finder Go menu provides a quick way to navigate there.
Type this command to check out the WebKit source tree:
svn checkout http://svn.webkit.org/repository/webkit/trunk WebKit
Once the checkout has finished, the next step is building WebKit.
Type this command to update your source tree:
WebKit/WebKitTools/Scripts/update-webkit
It's handy to put WebKit/WebKitTools/Scripts in your shell path so you can type commands like update-webkit without specifying a full path.
It is also possible to browse the SVN repository online at http://trac.webkit.org/project/webkit/browser.