LAS VEGAS -- Penn and Teller started out more than 30 years ago as exceptionally clever comic magicians. (Well, illusionists -- as they like to remind their audiences, magic doesn't exist.) Now, however, they've evolved into two of the country's most fearless cultural gadflies, the scourge of all things politically correct and socially misguided, taking to the nation's airwaves to say things that shouldn't be said, and to say them very loudly. Interestingly, they've mounted this assault from a home base here in Las Vegas.

It's a nice setup, actually. For one thing, they have their own theater, attached to a big hotel-casino called the Rio, right on the Strip, where six nights a week they do an elegantly staged one-hour show that intertwines comedy, magic, music, mime, juggling and occasional gunfire. Fifteen years ago, when the duo announced they were relocating here from New York, their fellow sophisticates were aghast. "When we told people that we were gonna do our show in Vegas," says Penn Jillette, the taller and more talkative of the two (Teller doesn't talk at all onstage), "it was like telling them, 'From now on, I'm working strictly in Day-Glo and velvet, and I'm gonna do nothing but Elvis and Christ.' "

Since that time, of course, Vegas has changed quite a bit. It's now an acceptable destination for all kinds of hipsters, from the Blue Man Group to Sarah Silverman. And, as Penn notes, "We have two bands here now too: Panic! at the Disco and the Killers. Before that, all we had was Slaughter." Still, it is Vegas -- doesn't the glitter overload ever get to them?

Not at all. Backstage after a show one night, having doffed their three-piece stage suits for shorts and jeans, they're happy to enumerate the town's virtues. "Vegas is a luxurious modern city," says Teller. (He legally dropped his first name, Raymond, three decades ago and now holds one of the few single-moniker U.S. passports). "There's every convenience: fine shopping, great restaurants, beautiful weather." In addition, he says, the Mojave Desert -- that vast, sun-baked expanse in the midst of which Las Vegas has risen up -- exerts a weird allure of its own.

"The desert's a really strange place," Teller says. "It attracts crazy people, and they leave their remains behind. I drove out one afternoon, really at random, and came to a hilltop on which there was this circle of burned-out cars, full of bullet holes. And in the middle of them, a dead burro. What could be a more perfect scene?"

"In the late '80s and '90s," says Penn, "everybody came to Vegas ironically. You flew in and you said, 'I'll have cigars, and I'll go see a sh---y show.' And you were purposely going to sh---y shows, and laughing at sh---y architecture. I didn't know anybody who went to Vegas without irony. Then what happened, I think, when we came in, and we kind of got the Blue Men to come in -- when that started happening, people would go to two sh---y shows and make fun of them, and then say, 'F--- it, I've gotta see a good show.' I would like to think that in 10 or 15 years, Vegas will be like New York, with people who live here, develop new material here and put it onstage."

Over the years, apart from buffing their act to its current high shine, Penn and Teller have frequently ventured into other fields: books ("How to Play in Traffic," "Cruel Tricks for Dear Friends"), movies ("Penn and Teller Get Killed"), radio (until last March, Penn had a syndicated show on CBS). And as zealous champions of the First Amendment and individual liberties generally, they have also lectured at places like Oxford University and the Smithsonian Institution, and are currently visiting scholars at MIT and research fellows at the Cato Institute, the libertarian think tank in Washington, D.C. It is this aspect of their interests that fuels their most provocative undertaking, a cable-TV series they started on Showtime* in 2003 called "Penn & Teller: Bullsh--!" (In an effort to conserve the endangered hyphen, we shall henceforth refer to this show as "BS.")

The series is conveniently produced and shot in Las Vegas, and it's a project into which the two men are able to pour all of their boundless skepticism about the human endeavor. Armed with copious research, rude humor, thematically relevant magic tricks and occasional flashes of T&A, they lash out mercilessly at all manner of organized pretense, cultural faddism and political nonsense both right and left. To date they've done scathing analyses of everything from alien abduction, feng shui and bottled water to gun control, conspiracy theories, the war on drugs (they're against it), the Bible (they're militant atheists), creationism (no thanks) and "environmental hysteria." There was also an episode intriguingly titled "Nukes, Hybrids and Lesbians."

They scoff at the notion of media "objectivity." On "BS," they're scrupulous about presenting both sides of every issue, but they also make it resoundingly (and often obscenely) clear which side they come down on. "The phrase we use," Penn says, "is 'Fair and Very Biased.' I think bias is terrific. You should know people's opinion. You should know that Greta Van Susteren [of Fox News] is a millionaire Scientologist. That's a piece of information you should have. It should crawl across the bottom of the screen."

Scientology would seem to be a natural subject for "BS," and Penn and Teller long contemplated tackling it. From the Showtime point of view, it was dangerous territory -- the Church of Scientology is notoriously litigious. After a 1991 Time cover story titled "Scientology: The Cult of Greed" portrayed the group as "a hugely profitable global racket," the magazine was quickly hit with a lawsuit. The case dragged on for 10 years, and while Time ultimately prevailed, the legal costs were astronomical.

But Penn and Teller weren't the only cultural guerillas with a knife out for Scientology, and in 2005, their friends Trey Parker and Matt Stone, the creators of "South Park," came up with a now-classic episode of their own show called "Trapped in the Closet" -- a double hit on Scientology and one of its most famous promoters, Tom Cruise. Penn was a little bummed about being beaten to the punch, but now he wonders how much "BS" really could have done with Scientology. " 'South Park' can play a cheerleader role," he says. "Like with Ouija boards, where everybody knows it's bullsh-- and you just join in. But I always like there to be some sort of revelation, some really surprising information. And everyone who's not a Scientologist already knows it's bullsh--."

"BS" got there first on another subject, though: PETA (People for the Ethical Treatment of Animals). Penn and Teller put their researchers to work on the group for the show's second season, and Penn says that at first he couldn't believe the reports they sent back. "I said to the researchers, 'Don't tell me the second-in-command of PETA [Senior Vice President MaryBeth Sweetland] is a diabetic who uses animal products [in the insulin she takes] to keep her alive, but thinks other diabetics should die, because since she's working for animals, it's for the greater good." (Sweetland's precise words, printed in a 1990 issue of Glamour magazine, were: "I don't see myself as a hypocrite. I need my life to fight for the rights of animals.") There was also the group's shady history of euthanizing thousands of the animals it claimed to be "saving." "We broke that story," Penn points out, rather proudly.

If it need be said, Penn and Teller are wholeheartedly in favor of animal-testing to create lifesaving new medicines. They're very big on science generally, and of course "BS" is a celebration of the scientific spirit of skeptical inquiry. Unfortunately, Penn says, "Hollywood and show business in general -- I don't know why this is -- are anti-science. Even Bono has this undertone of, 'The modern world is bad.' "

Without science, of course, we might all still be living in grass huts. And without skeptical inquiry we'd be defenseless against cyclical popular manias -- currently, the alleged horrors of such things as fast food, gay marriage and the ever-unpopular free speech. One wonders what the rationalists who founded this country would make of its current intellectual priorities.

"I was at an Eminem show," Penn says, "and someone asked me, 'What would Thomas Jefferson say about that?' I said, 'I know exactly what Thomas Jefferson would say. He'd say, "How do you get the roof to span this arena without supports? Where are the lights coming from? How is his voice that loud?" You wouldn't get to Eminem's lyrics until 35 hours into the conversation. First you'd have to answer Jefferson's questions about how the popcorn is made, and how is it just the right temperature in here. It would blow his mind. 'How do you keep the beer cold?' "

*(Like MTV, Showtime is a subsidiary of Viacom.)