For anyone fascinated by advertising-as-culture, I recommend an afternoon watching a NASCAR Sprint Cup race on TV. I also prescribe an overstuffed chair and a bowlful of M&M’s mixed with NoDoz.

One Sunday, during an interminable 400-miler from Dover, Delaware, I saw a commercial that made me sit bolt-upright in my Barcalounger, an ad that in its innocence and whimsy and desire for eyeballs would destroy one of the most valuable illusions of advertising.

That illusion: When it comes to celebrity product endorsements, celebrities get to lie.

During a live Web cast on October 20 NASCAR star Tony Stewart will be hooked up to a polygraph and posed the question: Does Tony really love Burger King’s Whopper sandwich? The Web cast will be the climax of a month-long campaign by Crispin Porter + Bogusky—“The Truth About Tony”—that seeks to demonstrate that Stewart actually, really, truly loves eating Whoppers.

The set-up spot, which is also in rotation, is called “Tony Stewart’s School of Endorsements.” The driver, in uniform, is in front of a classroom of C-list celebrities—Carrot Top, Erik Estrada—advising them on how to better their sales. Tony’s advice: “Endorse things you love.”

“Of course he loves Whoppers,” my wife sneered as she passed through the room. “I mean, just look at him.” (Stewart has a wee bit of a weight problem.)

Because it’s played for laughs, the BK campaign easily can be taken too lightly, but at stake is a fundamental dynamic in celebrity-based advertising. As Daniel J. Boorstin noted ages ago, celebrities are synthetic creatures, walking, talking, media-processed fictions that have little in common with the three-dimensional beings who share their names and bank accounts. And because they enjoy this semi-magical, almost folkloric existence, they can effectively endorse just about any product. We don’t expect them actually to be invested. The grammar of celebrity endorsement says, simply: Lie to us.

We instinctively know, for instance, that Sarah Jessica Parker is not running down to Target to pick up a bottle of Garnier hair coloring. You probably can’t get within 10 blocks of her with that stuff. And yet she happily appears in advertising for Garnier and the company happily pays the seven-figure freight to have her.

Does Brooke Shields use eyelash Rogaine? Does William Shatner negotiate his hotel rates on Priceline? I suppose it’s possible that rail-thin glam-giraffes Paris Hilton and Padma Lakshmi devour 1,000-calorie Carl’s Jr. burgers, but only if they’ve acquired tapeworms along the way.

This is—or has been—the way of things since before Buffalo Bill endorsed Spencer carbines. Nobody particularly cared if John Wayne smoked Camels or Bing Crosby drove Ford Thunderbirds. No one ever enforced a notion that celebrities have to love or use the products they endorse (although endorsement contracts stipulate that the party should avoid being photographed using a rival product). It was an understanding between the medium and the mediated.

But I wonder if the ground has moved and if Crispin Porter + Bogusky—with access to some of the best culture seismographs available—hasn’t picked up on it? Do consumers want to truth-squad celebrities?

Elizabeth Harrison, principal at Harrison & Shriftman public relations, told a Wall Street Journal panel earlier this year: “The worst is when [an endorsement] absolutely has zero integrity and you see a celebrity wearing something, doing something, going to something that makes zero sense for that celebrity.... It can really backfire on you, because the media picks it up, and particularly now, with all the blogs that are going on. Everybody’s much quicker to spot those things and to actually speak about them.”

In other words, the magnifying glass is bigger and therefore the focus on authenticity is sharper.

Meanwhile, the “accidental” product endorsement—in the form of a supposedly candid paparazzi shot of, say, Brad Pitt carrying a Starbucks latte or Simon Cowell wearing a Chopard watch—has been exposed as being almost always stage managed. In fact, paparazzi now will give celebs products to hold, in order for both parties to profit.

The result: The pleasant fiction between celebrity endorser and consumer has been badly eroded. Green Light, a talent and rights representation company, assayed the coverage of the recent 61st Primetime Emmy Awards broadcast and found a sharp decline in the percentage of celebrity endorsement ads. Green Light found that fewer than 6 percent of ads on the broadcast involved celebrities, down from 75 percent the year before. It’s partly a matter of cutting costs—celebrities are expensive—but it also seems to be an index of broken faith.

We’re living through a crisis of confidence on all fronts so it shouldn’t surprise us that consumers are growing suspicious of even our secular gods. But, if the Tony Stewart ads are any indication, celeb endorsers are going to have to put their mouth where their money is. Tina Fey better be whipping out that AMEX card. Michael Phelps better be spotted eating at Subway. Those guys in U2 had better have BlackBerrys on their hips.

Sarah Jessica Parker might even have to use Garnier. I would pay to see that.