Just read this on MacFixIt:
QUOTE
What gets installed by the Safari 3 Beta package Installing Safari 3.0 does much more than simply replace the Safari application (in /Applications) with a newer copy. It also does a transplant of the WebKit framework (relied upon by Safari and other applications to render page HTML content), the JavaScript framework, and -- surprisingly -- replaces the Dock with a new version. As such, you may experience problems with applications other than Safari after installing this beta release. One particular area (as noted below) concerns the operation of Widgets, which are rendered by WebKit.
Fortunately, Apple has included an uninstaller (Safari3BetaUninstall.pkg) that will revert all components replaced by the Safari 3.0 beta installer with the Safari 2.x associated versions.
Did any of you notice any difference in your Dock?
Not sure what MFI means by "replaces the Dock".