How do I get the white star back in top site in Safari 6.0 in Mountain Lion?
The Top Sites update indicator (blue curl with white star) used the RSS functionality (which if you remember was removed from Safari v6) to work - this is how it could tell if a website was updated.  This means that since Apple removed RSS for Safari v6, this also removed the Top Sites page site update indicator functionality.

Just like the removal of RSS for v6, this missing Top Sites indicator functionality is a feature of v6.0.