There’s a setting in the Preferences pane of Safari 4 that says “Open Links from Applications in ‘New Tab’ or ‘New Window.’ ”
Unfortunately, this doesn’t do anything when the link is already within Safari. Anytime a website tells the browser to open a new page, Safari’s default is to open a new window, NOT a new tab. For me, tabbed browsing is powerful because I DO NOT WANT multiple windows, just tabs, but soon, I start getting multiple windows with multiple tabs, and then I’m pissed.
Here is a quick way to make sure all new links open in a new tab (tutorial via switchingtomac.com):
defaults write com.apple.Safari TargetedClicksCreateTabs -bool true
Now restart Safari. Links that used to open in a new Window will now open in a new Tab instead.
If you decide you want to change back, just re-enter the command but replace true at the end of the command with false.