With the updated Safari Browser in Version 3.1 it’s now possible to prevent links to open new browser windows. This settings is available in Firefox browser for a long time. But it’s still not easily available for Safari. Although you can explicitly open a link in a new tab, you can’t prevent opening a new window and instead send it to a tab by default.
But with a little help from the Terminal you can achieve this goal easily:
defaults write com.apple.Safari TargetedClicksCreateTabs -bool true
found this a few days ago at fscklog