In the beta versions of Safari 4, the tabs were displayed in the title bar of the window. When Apple released Safari 4 final, they repositioned the tabs to the usual place since Most of the users didn't like the top tabs.
Anyway, I liked them very much. Is it possible to reenable the top tabs in Safari 4 final?
Unfortunately, this isn't possible.
The hidden preferences mentioned by others are no longer present in the final version of Safari 4, and it seems unlikely that another method is found. It appears Apple has discarded the tabs at the top for this version of Safari.
(The resources of the final version of Safari contain a file named TabsOnTopPreferences.tiff, but this is the only reference. The actual graphics for the tabs on top are no longer present.)