Thirty-eight years ago this week, Hair opened in London - one day after the Lord Chamberlain's power to licence plays had been done away with. And the hair that most obviously exercised the many journalists who covered the event was pubic. Hair, notoriously at the time, included a scene of mass nudity - the cast's own minor contribution to the banishment of shame.

It caused quite a fuss at the time, just as stage nudity did a couple of years later, when Ken Tynan's "erotic review", Oh! Calcutta!, opened at the Roundhouse. But since then, we tend to assume that the power of this kind of theatrical taboo busting has steadily diminished. True - the decision of a Hollywood celebrity to lay down her lingerie in the cause of art will still provoke a flurry of tumescent excitement in certain sections of the press - as was demonstrated by Nicole Kidman's appearance in The Blue Room and Kathleen Turner's towel-dropping tableau in a recent stage adaptation of The Graduate. But the days when a production could guarantee extra column inches with nudity has long gone.

Currently running at the Bush Hall in Shepherd's Bush, Kangaroo Court's updated production of Lady Chatterley's Lover includes explicit sex scenes and nudity, and yet, much as it might like a furore, it's having to make do without one. At the Royal Court, Kelly Reilly's topless appearance in Terry Johnson's new play Piano/Forte has provoked barely a ripple of improper interest. And in a couple of weeks, when the Barbican opens Ursula Martinez's performance Show Off, in which she strips naked twice, I doubt they are bracing themselves for excitement of the wrong kind.

On the face of it, we have arrived at that state of unflustered normality which the more ideological stage nudists always claimed they were trying to inaugurate. What's the fuss, after all? It's perfectly natural... we're all grown-ups here. Except that you only have to attend a play in which nudity occurs to discover that we're not - or at least that our adulthood remains thoroughly compromised when it comes to this subject.

Take Reilly's scene in Piano/ Forte. She plays the wild-child daughter of a disgraced Tory MP, returning home to vandalise his forthcoming wedding to a Page Three girl. After establishing her credentials as a bohemian fury, she disappears upstairs just before the new wife arrives, and when she comes back down again to be introduced, she is naked from the waist up - a fact she pretends not to notice as she launches into brittle small talk. As a form of sabotage, it is comically effective, but what was notable on the night I saw it was how much it thrilled the audience - not so much sexually as theatrically. It was a coup - because we'd momentarily been pulled out of the play and then snapped back into it, like a rubber band stretched taut and then released.

Nudity on stage has a tendency to do this - since the character being played is briefly eclipsed by the exposure of the actor or actress. What people were thinking in Piano/Forte was not, "Oh look, Louise is flashing her tits", but, "How bold of Kelly Reilly" and, probably a millisecond later, "How do the breasts stand up to scrutiny?" (With pert assurance, if you're interested. I don't want to sound like some panting old roué but her nightly exposure can only add to her reputation as one of the sexiest actresses currently on stage).

And this essentially anti-theatrical impulse needn't always be at odds with what the theatre wants to achieve. Johnson, a playwright with some previous when it comes to the galvanising properties of nudity (his Hitchcock Blonde featured a naked shower scene), surely understands the short circuit of intimacy it delivers - a sparking contact not between a fiction and an audience that has temporarily lost itself in narrative but between a performer and an audience all too aware that it is seeing something it usually wouldn't.

That this effect should have survived the past four decades, during which nudity has become a routine staple in advertising and film, is perhaps a little surprising, but it's also a testimony to the unique intimacy of theatre, a place where the exposure of the actor - bodily and emotional - can't be protected by retakes or retouching. When Sadie Frost strips off to protest against the use of animal fur in fashion, we can be pretty sure that someone stands between us and her human frailty. When an actor or actress does it, they bare themselves in quite a different way - and however adult we are about it, we're unlikely to be immune to the charge it delivers.