Fashion superstarTom Ford also appears on the cover photo, though he stuck with a more traditional suit -- one of black fabric.
Ford, the issue's guest art director, said he hadn't planned on becoming part of his own project, but he stepped in when "Wedding Crashers" star Rachel McAdams, 29, backed out.
"She did want to do it, and then when she was on the set I think she felt uncomfortable, and I didn't want to make anybody feel uncomfortable" Ford said Tuesday on ABC's "Good Morning America."
Other Hollywood stars weren't difficult to persuade, Ford said.
"A lot of women actually, a couple of men, too, wanted to take their clothes off," he said. "These are such beautiful people, beautiful women, and who doesn't want to see a bit of them."
Angelina Jolie also got naked for the Hollywood issue, posing in a bathtub. Another shot depicts a fully clothed George Clooney as a director filming a large cast of women wearing skin-tone underwear.
For the cover photo, Knightley, 20, was photographed sitting sideways, with legs and arms carefully placed. The 21-year-old Johansson, draped over Ford's leg, is somewhat more exposed -- readers who open the fold-out cover will see her bare buttocks.
Knightley is currently nominated for a best actress Oscar for her performance in "Pride & Prejudice." Johansson co-starred this year in Woody Allen's "Match Point."