"I'd like to dedicate this song to Britney," Madonna said, pacing the stage of New York's Roseland Ballroom Sunday night in a sleek black rhinestone-cowgirl outfit with a Britney Spears T-shirt peeking out from underneath.

Her voice was soft, and the unexpected statement seemed straight, not satirical. Then, backed by a small, keyboard-heavy band led by her latest creative collaborator — the French-Afghani dance-pop producer Mirwais Ahmadzai — she launched into "What It Feels Like for a Girl", the sweetest of the 10 tracks on her latest, double-platinum album, Music.

It was one of five new tunes she performed in a free, half-hour show for radio contest winners and friends such as Gwyneth Paltrow, Rosie O'Donnell and Donatella Versace at the venerable Manhattan music club, which she'd rigged out for the occasion as a sort of country-disco salon. A big, glittery disco ball twirled above the dance floor, carousel-style horses and hay bales, and cowboy murals filled the rest of the room, in keeping with the graphic motif of the new album.

Madonna hasn't toured for seven years, and even then, with her Girlie Show, she only played two U.S. cities. But while she claimed to be nervous about being back in the spotlight again ("It's been a while!"), she seemed loose (tumbling backward into the audience), playful (popping up in the back of an onstage pickup truck) and pretty chatty, too.

"The last time I was on the stage at Roseland," she told the crowd, "was about 18 years ago. I was the opening act for New Edition, and I just wanna say — if Bobby Brown's in the audience — I have a bathrobe with my name on it now, too."

The Roseland show was a concentrated demonstration of pure charisma; it may have been brief, but Madonna owned every minute of it, whether hoofing a faux two-step with her similarly cowgirl-clad backup team (the invaluable Niki Harris and Donna Delory) or tearing through the slam-bang finale, a deconstructed rendition of her latest #1 single — Music's title track — which was keyed to a frenetically edited and altogether eye-popping video montage that referenced just about her entire stylistic history.

It's unfortunate that so few people got to see this one-off show, and sad to think that by now there is practically a whole generation of pop fans who've never seen this singular performer play live. It's tempting to say that Madonna would vaporize the competition, but there really is no competition: In the absence of a touring commitment, she's simply relinquished the field to her inferiors.

Will she ever tour again? Couldn't the Roseland gig, with some creative inflation, be fitted out as a full-scale road show? A little later Sunday night, at a downtown scene-bar called Eugene (where her Maverick Records partner Guy Oseary was celebrating the publication of a book he'd put together called "Jews Who Rock"), Madonna — in another Britney Spears T-shirt, this one punk-chopped and safety-pinned — was contemplating the ever-recurrent touring question with her friend Ingrid Casares, the Miami club-owner, while such celebs as Ben Stiller and Sandra Bernhard milled around among the sofas and side tables. Asked if another tour will ever happen, Madonna grinned and said, "I'm thinkin' about it!"

We've heard that before. But I guess it's good to hear it again anyway.