Safari 4 changes the locations of the tabs to be more like the tabs in Chrome by putting the tabs above the windows (above the bookmarks bars, address bar, search bar, etc). If you prefer them back down below all of this and directly above the main part of the window ala Safari 3, you can change this setting.

To change it you'll first need to Quit Safari and then open up a terminal window. In the terminal window, enter this command:

defaults write com.apple.Safari DebugSafari4TabBarIsOnTop -bool FALSE

You can then restart Safari 4, and the location of the tabs will be moved. If you change your mind and want to move them back, you can repeat the command and change the FALSE to TRUE.