Penn's acclaimed performance in Focus Features' Milk as the late San Francisco gay rights figurehead Harvey Milk shut out Fox Searchlight's Golden Globes champion Mickey Rourke and sets up an intriguing Oscar night showdown with The Wrestler star.
Streep's best lead actress for her role as the inquisitorial Sister Aloysius Beauvier in Miramax's drama Doubt dashed Kate Winslet's chances of repeating her double triumph from the Golden Globes.
Winslet's extraordinary awards season kept going however as the British star claimed the best supporting actress prize for The Weinstein Company's German drama The Reader. She had also been nominated in the lead actress category for DreamWorks' suburban period drama Revolutionary Road.
Heath Ledger maintained his posthumous run for an Oscar as the late actor was named best supporting actor for Warner Bros' crime epic The Dark Knight.
James Earl Jones received the lifetime achievement award and earlier in the day the stunt performers and coordinators from The Dark Knight were announced on the red carpet as winners of the Screen Actors Guild's award for feature stunt ensemble.
15th ANNUAL SCREEN ACTORS GUILD AWARDS WINNERS
Theatrical Motion Pictures
Outstanding Performance by a Cast in a Motion Picture
Outstanding Performance by a Male Actor in a Leading Role
Sean Penn Milk
Outstanding Performance by a Female Actor in a Leading Role
Meryl Streep Doubt
Outstanding Performance by a Male Actor in a Supporting Role
Heath Ledger The Dark Knight
Outstanding Performance by a Female Actor in a Supporting Role
Kate Winslet The Reader
30 Rock - Outstanding Performance By An Ensemble In A Comedy Series
Mad Men - Outstanding Performance By An Ensemble In A Drama Series
Alec Baldwin for 30 Rock - Outstanding Performance By A Male Actor In
A Comedy Series
Tina Fey for 30 Rock - Outstanding Performance By A Female Actor In A
Hugh Laurie for House - Outstanding Performance By A Male Actor In A
Sally Field for Brothers & Sisters - Outstanding Performance By A
Female Actor In A Drama Series
Paul Giamatti for John Adams - Outstanding Performance By A Male Actor
In A Television Movie Or Miniseries
Laura Linney for John Adams - Outstanding Performance By A Female
Actor In A Television Movie Or Miniseries