I couldn't see a whole lot of changes after installing the 10.3.9 update to Mac OS X, it looks as if a lot has changed in WebCore and Safari, in particular:

• HTML editing API in both Objective C and HTML (via contenteditable and designMode).
• Full DOM level 2 support, including APIs from Objective C.
• XSLT support in Safari

Pretty cool. Hopefully Tiger will have some tighter networking code to make the whole sheebang faster.