Stealth advertising campaigns seek to create viral videos that have no direct reference to the sponsoring brand. And product placements in movies and on television meld advertising into the content of traditional entertainment fare.

While these techniques seek to slip commercials past the audience so they may not be aware they are watching an actual ad, an alternate approach seeks to engage the audience in the ad campaign itself. Call it “advertising as entertainment”: A product campaign that’s so much fun you actively seek it out.

One could, of course, argue that on some level all good advertising seeks to entertain. Indeed, there’s genuine excitement in watching Apple’s ‘1984‘ Superbowl ad to introduce the Macintosh or Eminem touting Chrysler automobiles by declaring “This is the Motor City. And this is what we do.” And the witty print ads for the Volkswagen Beetle by Doyle Dane Bernbach in the 1960s certainly provided moments of amusement.

The new approach to advertising as entertainment, however, ratchets up this idea to a new level. It uses the techniques of an emerging form of art and entertainment — known as “alternative reality gaming” — that combines content from multiple media with real-world interaction. The audience becomes an active participant in the advertising. Just as it is engagement that makes social media effective, “engagement advertising” may bring new life to promotional campaigns.

A cross-promotion for Marvel Studios’ forthcoming motion picture Thor and Acura automobiles is one recent example of this trend. The campaign launched broadly on April 11, 2011 with a series of television, online and print advertisements. But the proto-campaign — the build up to the launch — began as a series of interactive events that used transmedia techniques to build buzz for the forthcoming campaign.

Wharton’s director of new media, Kendall Whitehouse, recently attended the San Francisco comic book, science fiction, and motion picture convention known as WonderCon. There, the ad campaign for Thor and Acura literally came knocking on his hotel room door.

A messenger delivered a mysterious card bearing the logo of S.H.I.E.L.D. — the fictional spy agency in the Marvel comic book universe — with instructions to “Report to channel 72.” When the TV on the room was switched to that channel, a flood of cryptic images appeared, ending with the address of a web site. The website contained a single page, thanking him for “considering global employment opportunities at Strategic Homeland Intervention, Enforcement and Logistics Division” and requesting his email address.

Outside the hotel, additional elements of the extensive campaign were in evidence. Between the hotel and the convention’s home at the Moscone Center, a line of people were queued up heading into a tent patrolled by figures in military-looking garb. They were waiting to apply for their own S.H.I.E.L.D. ID badge – and to agree to receive additional information from S.H.I.E.L.D. and Acura.

Whitehouse’s entire experience, along with photos, can be found in On Technology and Media: “Advertising as Entertainment.”

In a world where people switch the TV channel the instant a commercial appears, here was a group of individuals lined up to participate in an advertising campaign – the details of which were deliberately obscure. Welcome to the future of marketing: advertising as entertainment.