Safari 6 was released on July 25, 2012 as an update to OS X 10.7 (Lion) users, and as part of OS X 10.8 (Mountain Lion).
There is no longer an easy menu choice to empty the cache. Previous to Safari 6, it was simply Safari -> Empty Cache.
In Safari 6:
1. You first have to enable the Develop menu:
Safari -> Preferences -> Advanced (icon) -> (check) Show Develop menu in menu bar
2. Then Develop -> Empty Caches