Summer officially is here at the domestic B.O., as Paramount and Marvel Entertainment’s “Iron Man 2″ scored an estimated $133.6 million in its opening weekend.The 2D comic book sequel, which now ranks as Par’s highest-grossing domestic opener, took in an additional $57.2 million in 61 overseas markets on 10,774 screens, boosting its worldwide haul to a stellar $327.6 million. The film bowed internationally last weekend in 53 territories, marking an unprecedented move among summer tentpoles to kick start the season first overseas. The decision paid off as “Iron Man 2″ became the fifth-highest three-day opener ever domestically, surpassing “Shrek the Third,” which debuted with $121.6 million in 2007. Opening in limited release opposite “Iron Man 2″ was Focus Features’ Mother’s Day-positioned docu “Babies,” with a solid estimate of $1.6 million at 534 locations. The docu scored a per-screen average of $2,951.