I’ve done a bit of searching for specific Safari version numbers in support.apple.com. Quite a few versions have specific change notes, but it seems like often the versions bundled with major OS X releases aren’t given their own change notes.
1.2: http://support.apple.com/kb/DL612 (Dave Hyatt also provided some more detailed notes on the old WebKit blog: http://weblogs.mozillazine.org/hyatt/archives/2004_02.html#004874)
Also, sometimes Apple’s release notes can be a bit... terse.
I’m trying to figure out which versions of Safari introduced support for HTML5/CSS features.
Wikipedia’s got a great table of releases, but it is a bit vague on CSS improvements (e.g. “improved CSS support”).