Most nights, it wouldn't seem particularly weird that the guy stamping hands at the door of CBGB's 313 Gallery in New York was toting a pack of Marlboros. But at "The Billboard Liberation Show" -- celebrating two decades of guerrilla art attacks on outdoor ads, above all cigarette billboards -- there was a touch of irony in the gesture.

Last Wednesday night, the gallery was packed with downtowners -- dressed-for-the-office young women with pierced eyebrows, a guy bustling around with a Starbucks sticker on his head apropos of, um, some statement -- viewing more than 1,000 adulterations of outdoor signage by the Billboard Liberation Front (BLF), Cicada, Ron English and other practitioners of "culture jamming," put up temporarily and photographed for posterity. (Those unable to catch the show, through May 21, can find many works at the BLF home page -- which, fair warning, includes one of the most irritating audio files on the Internet.)

The timing couldn't have been better, considering that not two weeks earlier, cigarette billboards from coast to coast were replaced with anti-smoking ads as part of the state tobacco settlement, effecting perhaps the country's largest act of billboard liberation -- and depriving these very artists of their favorite targets. (The exhibit is riddled with Joe Camel desecrations -- Joe as a dinosaur, Joe in a coffin, etc.) Or it couldn't have been worse, for the same reason. With the Marlboro Man supplanted by state-sponsored adbusting, with ad alteration becoming an-ever-more-popular amateur pastime, with Conceptual artist Barbara Kruger having done ads for the Economist, what's a culture jammer to do nowadays?

In 1977 the Billboard Liberation Front in San Francisco performed the first of what it calls billboard "improvements" on a Max Factor ad, changing the billboard slogan "A pretty face isn't safe in this city. Fight back with Self-Defense" to "Fight crap with Self-Respect" and "Fight back with Self-Abuse." Over the years the group -- which doesn't espouse a political agenda so much as a playful humor and the belief that "everyone should have their own billboard" -- refined its methods, crafting elegant overlays to plaster on signs at night, and later moving on to electric signage. (In a gesture of solidarity, the BLF leaves behind 12-packs of beer for the billboard workers who clean up after them.)

If you read alternative weeklies, you know where it goes from here. Other groups and guerrilla artists began making raids across the country, some heavy-handed ("New World Order" over Picasso's "Guernica"), some whimsical ("Masturbation Is Murder"). By the '90s, articles on billboard pranksters -- the artist stealing out at midnight, the quote about the "commercially-induced 'American Dream,'" the flustered response from some middle-management advertising straw man -- were as reliable an alternative-rag fixture as "Life in Hell" and the top ten censored stories report.

Billboards, as BLF notes, are probably the only major medium created for and dedicated to advertising; and billboard alteration is a distinctively American art/protest form, equal parts Jenny Holzer and Ogden Nash. Outdoor ads are ubiquitous, often invasive, and they're also our own public art, our capitalist frescoes; the artists here found a way to make relevant, accessible art by turning a massive monologue into a conversation.

A good billboard alteration is jarring, creepily hilarious ("If you're looking for a sign from God, here it is. Kill yourself"). A great one can be thought-provoking, even beautiful: A Smokey the Bear ad with a giant lit match becomes a haunting memorial to the Los Angeles riot: "Amber Waves of Flame: LA '92." The worst are sophomoric or drip contempt for all those brainwashed robots filling their big suburban houses with crap: "WARNING: Sabotaged propaganda may be dangerous to apathetic minds." (Does my art shock you, sheep?) It's an attitude that -- surprise -- cropped up in some of the CBGB audience. "They could only get away with this in New York City," a man purred to his date. "In Kansas they'd get arrested."

The earlier works, from when ad-busting was less of an AlternaWorld fixture, are the more vital and arresting, up through Artfux's searing Gulf War protests ("I WANT YOU to die a horrible, meaningless death to sustain a lifestyle that will ultimately destroy the Earth"). Today, the enterprising ad-buster competes with advertisers parodying themselves, intentionally or unintentionally. (Last year, just two blocks from CBGB, Combat roach traps put up a billboard with the Grim Reaper and the slogan "Got Death?" spoofing the famous dairy campaign -- obliviously, directly above the Buen Pastor Funeral Home.) You find ever more artists riffing on a few usual suspects: "Think Different," Joe Camel, Marlboro.

For instance, Charles Manson's image, the ketchup of ad alteration, is imposed on both Levi's and Apple billboards. (The BLF's online how-to-parody manual lists ad-buster chestnuts like so many household spices: "Some ads lend themselves to parody by the inclusion of a small image or symbol in the appropriate place [a skull, radiation symbol, happy face, swastika, vibrator, etc.].") Well, we all know Manson is way transgressive. But what statement does this make, exactly? Levi's and Apple have used dead historical figures to plug their products. Some dead historical figures were good. But others -- aha! -- were evil. And ... and ...? In the San Francisco Examiner, D. S. Black called it a reaction to Calvin Klein's heroin chic, a Debordian detournement. It comes off, though, as frustrated flailing at monoliths -- at just everything, you know, that everybody's always trying to sell you.

The show's best exhibitor, actually, is a media artist who doesn't work in billboards: hoaxster Joey Skaggs, who in 1994 passed off a press release claiming to be from a Korean company canning dogs for food ("Dog no suffer. We have quick death for dog") and in 1995 burned CNN by posing as a scientist who had invented a computer program that found O.J. Simpson guilty -- playing into whites' longing for a bloodless rebuttal to the "irrational" Simpson jury. Skaggs shows a complex understanding of the workings, assumptions and prejudices of modern media and society, something a "Got Sperm?" billboard just doesn't quite nail. (Today, some online sites like Chickenhead are also doing especially sharp parodies.)

But if some of the liberations are old hat, two decades of plugging made them that way. Hammering Big Tobacco is pretty much a two-foot putt now, with everybody from advertising pros to schoolkids getting into the act; with the New York Times announcing last week it would refuse tobacco ads; with state-sponsored anti-smoking messages indistinguishable from counterculture pranks (one shows a cowboy telling another, "I miss my lung"). It wasn't so common in 1977, when the BLF targeted the weirdly named Fact cigarettes, changing a billboard's text from "I'm realistic. I only smoke Facts" to "I'm real sick. I only smoke Facts," with an arrow pointing from "Facts" to the Surgeon General's warning.

You could perhaps see the mixed results in the CBGB crowd, which was mostly in Garanimals back when the BLF started out and has since absorbed commercial skepticism like many Americans have: which is to say, kind of. Lingering around the walls full of doctored booze ads and defiled Joe Camels, the young rebels with thick-framed glasses -- just like admen wore in the '60s -- flirted, drank beers and smoked, one cigarette after another after another after another.

"The Billboard Liberation Show" is at CBGB's 313 Gallery, 313 Bowery, New York, though May 21, 1-6 p.m.