It's always a little embarrassing when someone notices that the thing you're treating like trash is actually a treasure. So Philadelphia should be feeling properly chagrined that it took the National Trust for Historic Preservation to point out that the endangered Boyd Theater is an architectural gem worth hanging onto with all our municipal might.

The art deco extravaganza on Chestnut Street is back on the market, six years after the city's Historical Commission declined to intervene in a developer's demolition plans and award the building historic status. The only reason it stands today is that the current owner, Live Nation, came to the rescue, convinced that the old movie house would make a spectacular new musical theater.

Live Nation's Larry Magid says he still believes the Boyd has star potential, but his company has decided, for internal reasons, to bow out of the theater business and sell it.

And that puts the Boyd right back where it was in 2002. Thanks to the killer combination of passivity and politics, Philadelphia's last surviving downtown movie palace still lacks the protective mantle of historic certification. What's changed is that the Boyd's location, on Chestnut Street's blossoming 1900 block, is now catnip for developers.

Live Nation is giving bidders an extended deadline of Wednesday, instead of tomorrow, to submit offers for the 2,350-seat theater and an adjacent parking lot. It's entirely possible that a sympathetic theater operator could triumph over an ambitious condo developer. But since Live Nation will need time to evaluate the bids, we may not learn the Boyd's fate for days, or even weeks.

Why wait to find out? The nominating petition necessary to start the historic-designation process remains stored on the hard drive of computers at the Preservation Alliance.

Submit it now - before the next owner has a chance to apply for a demolition permit.

Cranking up the designation process won't guarantee that the 1928 theater will win historic certification, but many are convinced it could qualify in a snap.

A formal nomination for city designation would freeze the building's status quo. And that's a whole lot better than the alternative.

It was the lack of formal certification that prompted the National Trust to include the Boyd on this year's list of America's 11 most important endangered historic places, says Adrian Scott Fine, the trust's local representative and a member of Friends of the Boyd. In Philadelphia, any building lacking local designation is fair game for the wrecking ball, even if it's listed by the federal government on the National Register of Historic Places.

Think back to spring 2002, the last time preservationists attempted to have the Boyd certified.

Then-owner Ken Goldenberg, who contributed almost half a million dollars to John Street during his two mayoral terms, arrived at the Historical Commission's designation hearing with feared über-attorney Richard Sprague in tow.

At that point, it didn't matter that the Boyd was the one of the last intact representatives of about 430 movie theaters built in Philadelphia before the Great Depression, during Hollywood's Golden Age. Or that the Boyd was the first in the area to install a wraparound Cinerama screen. Or that its jutting deco crown, glass retail arcade, and original ticket kiosk were beloved landmarks.

The Historical Commission certainly didn't care about the quality of the interior, with its etched mirrors and its vivid polychrome frame over the stage - Philadelphia's preservation laws don't cover interiors.

On that day in 2002, the commission wasted no time in dismissing the nomination. One member, explaining why he voted against historic status for the Boyd, said the theater was "an old, decrepit, falling-down disaster."

What neglected historic building isn't?

Even now, some preservationists are hesitant to push the Boyd nomination. They fear that certification could scare away buyers, or prevent the next owner from finding a creative way to preserve the theater. There may also be concern that a designation could be overturned in a lawsuit, as happened in 1993 after former owner United Artists complained that historic status was based on the theater's interior decoration.

But if you give in to those fears, why bother with historic preservation at all? By nature, preservation laws are restrictive and bothersome to private property owners. We tolerate them because our society believes that, at some point, certain structures enter the public domain, at least for discussion purposes.

What's the point of having a Historical Commission that won't protect important buildings, that won't create historic districts, that won't utter a word of protest when a bullying state agency razes a row of architecturally important buildings on City Hall's doorstep? This isn't just about the Boyd; it's about what Philadelphia will make of itself in the 21st century.

More than almost any other city in America, Philadelphia has much to gain from its older buildings. This is one of the last places where time is rendered visible, in the crazy-quilt glory of our streets. That authentic mix remains one of the most powerful reasons to visit, and to live, here. Retaining the past doesn't mean that nothing new can ever be built, but only that interventions should be thoughtful.

Nearly every sizable city in America has found a way to preserve at least one great theater from Hollywood's heyday, Fine says. That includes metropolises less well-heeled than Philadelphia, such as Detroit and Birmingham, Ala.

Compared with those rescue projects, the Boyd should be easy, since it's been shuttered a mere six years.

Almost certainly, political will and public money will be needed to save the theater. The Nutter administration still appears uncertain how to respond to the National Trust's listing. Though a statement was promised yesterday, there was no word from City Hall as of early evening.

The Trust meant to give Philadelphia a gentle prod, to get it to do right by the Boyd.

The real embarrassment will be making the list of cities that allowed their last great movie palaces to fade to black.