It has been noted at other places: The tabs in Safari 4 appear in the very top of the window, where you’d display page information and could CMD-click to see the browsing history. And you could easily move the window with a click ‘n drag. That space is now (mostly) occupied by the tabs. To go back to what it used to be previously just do this (at your own risk):
defaults write com.apple.Safari DebugSafari4TabBarIsOnTop -bool NO
There you are, the tab are back just above the content (where I think they belong anyway).