The result consolidated the Marvel Studios adaptation as the season's first bona fide blockbuster and relegated new openers Speed Racer from Warner Bros and What Happens In Vegas from Fox to second and third, respectively.
Box office was up 21% against the same weekend last year as the top 12 films combined for $128m. Year-on-year 2008 still trails 2007 by some 2.5% at this stage in the game.
Speed Racer disappointed on $20.2m. The family film was adapted from the 1960s Japanese anime television series by Andy and Larry Wachowski in their first outing as directors since The Matrix Revolutions opened in 2003.
Emile Hirsch stars as a talented racing driver in a high profile cast alongside Christina Ricci, John Goodman, Susan Sarandon and Matthew Fox.
Opening in third place on $20m was the romantic comedy What Happens In Vegas with Cameron Diaz and Ashton Kutcher. By the time actual results are confirmed on Monday, What Happens In Vegas and Speed Racer may switch places.
The remaining films each dropped two places except Redbelt, Sony Pictures Classics' tale of a retired martial arts master forced back into competition, which climbed 31 places to tenth in its second weekend. David Mamet wrote and directed the action starring Chiwetel Ejiofor.
Next weekend's wide release is Buena Vista's The Chronicles Of Narnia: Prince Caspian starring newcomer Ben Barnes.
Estimated Top 10 North America May 9-11, 2008
Film (Dist)/Int'l dist/Est wkd gross/Est total to date
1 (1) Iron Man (Paramount) PPI/Various $50.5m $177.1m
2 (-) Speed Racer (Warner Bros) WBPI $20.2m -
3 (-) What Happens In Vegas (Fox) Fox Int'l $20m -
4 (2) Made Of Honor (Sony) SPRI $7.6m $26.3m
5 (3) Baby Mama (Universal) UPI $5.8m $40.4m
6 (5) Forgetting Sarah Marshall (Universal) UPI $3.8m $50.8m
7 (4) Harold And Kumar Escape From Guantanamo Bay (New Line/Warner Bros) NLI
8 (6) The Forbidden Kingdom (Lionsgate/TWC) TWC/Mandate
Int'l/Relativity $1.9m $48.3m
9 (7) Nim's Island (Fox) Summit Entertainment $1.3m $44.3m
10 (41) Redbelt (Sony Pictures Classics) SPRI $1.1m $1.2m