From memory, I hold the control and/or shift key when clicking on the link to force it to a tab.
I use Firefox (with my XP machine) with tab browser extension which has a single window mode option. This forces new windows to appear in the one window.
On my iBook, I set Safari to use tabs, but new windows still pop up (from some links).
Is there a way to force new windows to appear as tabs within the single Safari window? (yes I know that I can click to have the link open as a new tab, but I'd prefer to have this the default option, rather than haveing to remember to hit the key combination to open the link as a new tab).