by Steve Ryfle In this day and age, few celebrities are shy about baring it all onscreen. So when bashful Natalie Portman asked director Mike Nichols to cut her nude scenes from the upcoming flick "Closer," it was newsworthy. Portman -- for now, at least -- is that rare star who chooses to keep herself under wraps, while much of Hollywood, male and female, is dropping its drawers.

Almost every actress currently on the A-list has done a nude scene or two. Some of them did it a few years before becoming mega-famous, like Reese Witherspoon ("Twilight") and Catherine Zeta-Jones ("The Mask of Zorro"). A few actresses used an attention-grabbing nude scene to help catapult their careers -- witness Halle Berry ("Monster's Ball") and Kate Winslet ("Titanic") -- while others have continued taking it all off long after their star status was cemented, like Angelina Jolie ("Taking Lives") and Nicole Kidman ("Cold Mountain," among others). And then there was Meg Ryan, trying to rekindle her career by letting it all hang out ("In The Cut").

"Halle Berry proved that one could garner high accolades while getting down and dirty with her naked sex scenes in 'Monster's Ball'," says Mr. Skin, otherwise known as James McBride, a movie nudity expert who runs MrSkin.com and author of an upcoming book, "Mr. Skin's Skincyclopedia."

"She garnered an Academy Award for performance, a quarter-million bucks for each chest-berry she bared in her follow-up flick 'Swordfish,' and she seems to have lit a fire under other top-tier stars who have peeled down on screen in her wake."

Even some of Hollywood's classiest and classic ladies, such as Kathy Bates, Meryl Streep, Diane Lane, Helen Mirren and Sissy Spacek, have worked in the buff. And we haven't even started talking about the guys yet. Three movies currently in theaters, "Alexander," "Sideways" and "Kinsey," feature full-frontal male nudity. And there are bare boys' bums galore: George Clooney, Tom Cruise, Brad Pitt, Denzel Washington, Ewan McGregor and others have all been unafraid to drop trou.

Sex in mainstream American movies has long been controversial, but today's stars appear to have left old taboos at the door. All that could change, what with the increasingly conservative political climate. But for now, are we witnessing a high-water mark in Hollywood nudity?

Not really, says McBride -- It just seems that way.

"It's definitely a momentary thing. In the summer everything is PG-13, because the people who go to the movies are kids. Then fall comes, and you get all of your R-rated movies and all your movies with nudity."

What are the benchmarks by which all nude scenes are measured? McBride says the first on-camera nudity by a major celebrity occurs in the "Ecstasy" (1932), in which Hedy Lamarr skinny-dips "and her chest is clearly visible," he says. Another historic scene, he says, is the wet-dream sequence in "Fast Times at Ridgemont High," when Phoebe Cates takes off her bikini top. "In my skin-pinion, all previous celebrity nude scenes led to that moment and its greatness looms permanently over all that have come since."

Filmmakers first photographed the human body sans clothing way back in the silent era, but it wasn't until after the Production Code was lifted in 1967, ending the movie industry's 30 years of self-imposed censorship, that sex on the big screen became commonplace. About the same time, European films full of graphic female nudity (try renting Jean-Luc Godard's "Contempt," which opens with a l-o-o-o-n-g shot of Brigitte Bardot in the buff) were drawing critical acclaim and inspiring young American directors.

Thirty-odd years ago, scenes of Jon Voight and Brenda Vaccaro romping around in the sack in "Midnight Cowboy," or Marlon Brando using butter as a sexual aid in "Last Tango in Paris," were pushing the envelope; today they seem rather tame. The explosion of porn, and the prevalence of T-and-A in movies and TV, seem to have desensitized audiences to the sight of nekkid flesh.

But that doesn't mean that people aren't still titillated when their favorite Hollywood stars take it all off. Just ask McBride, who's turned a teenage obsession into his website, which has become a multi-million dollar business with 30 employees (McBride frequently appears on Howard Stern and other shows, doling out his celebrity Anatomy Awards). And Mr. Skin isn't the only authority on famous people caught with their pants down. There's also Celebrity Sleuth magazine (the granddaddy of this particular obsession) and the Celebrity Nude Database (www.cndb.com), to name a few.

And when it comes down to it, the fixation with celebrity flesh is, not surprisingly, a mostly male hobby, says Craig Hosoda, author of the Bare Facts Video Guide (which lists the minute that a nude scene occurs, so you can fast-forward to it).

"We see (celebrities) so much on the movie screen or on our television screens that we feel a personal connection with them. They are almost like our friends. As a hobby, most guys mentally undress women. Whether she's a co-worker at work, a teacher at school, the checker at the supermarket or another shopper at the mall ... The chances of seeing one of these women naked is probably less than seeing his favorite actress naked. Kind of a weird phenomenon when you think about it."