Im using safari 4 Beta on Snow Leopard (yes, the one with tabs on top) and decide to mod safari 5 with the Original tabs from the 4 beta, it looks swell…

To install
Backup the original files before you move the new files
into the iTunes Resources folder.

Control/Right-Click Safari.app and select Show Package,
Navigate to Contents –> Resources
Drop the files into Resources folder including the Extras (optional)