Safari (by default) will open certain links (e.g. target=”_blank”) in a new browser window instead of a new tab even if you have your preferences set no too. Fortunately, there is a hidden preference you can set to force Safari to behave.
If you run this command in Terminal /Applications/Utilities/Terminal.app
defaults write com.apple.Safari TargetedClicksCreateTabs -bool true
to disable this afterwards, change the true on the end to false.
I have tested this on Safari Mac 2, 3 and 4 Beta.