Safari opens new window all the time …
but there is a solution. I found it on http://www.tuaw.com/2008/09/08/terminal-tips-force-safari-to-open-all-links-in-new-tab/
Just open Terminal (/Applications/Utilities/) and type in the following command:
defaults write com.apple.Safari TargetedClicksCreateTabs -bool true
Once you restart Safari, all clicked links will open in a new tab. If you want to reverse the command for some reason, just replace “true” with “false.”
It also works for Safari 4.0.