That update doesn't reinstall 10.6.8 - it just updates it, including a new Safari version.
Assuming you installed the update, not reinstalled the OS - if Safari is in your HD/Applications folder, it should be 5.1.8.
Open the Applications folder directly from Finder - select Safari & use Finder-File-Get Info (Command I))
what version does that show?
If 5.1.8 - you maybe have an older version elsewhere, which you should remove & then drag the new one to your Dock instead.