Apple’s documentation is pretty light on what’s changed.
Safari version 2.0.1/1.3.1 for Mac OS X Tiger/Panther improves website compatibility, application stability and support for 3rd party web applications.
It sounds like WebKit might have been updated (perhaps to include Acid test compatibility). It could also be that minor parts of the interface were amended, but I didn’t find any such changes. If you use Saft, you’re going to have to update that too.