Getting David Letterman to shill for CBS in the Super Bowl is tougher than a Stupid Pet Trick.

In each of the last two CBS broadcasts of the Big Game, Mr. Letterman has surprised viewers by turning up during one of the game's commercial breaks to do what is known in the TV business as a "promo" -- essentially a commercial for the network transmitting the program. The spots, which aired in 2007 and 2010, have proved to be delightful surprises that outshone many of the high-price commercials surrounding them. In the first, Mr. Letterman was seen watching the Super Bowl with Oprah Winfrey, with whom he has had an on-again-off-again friendship (and feud) of sorts.

In the second, he appeared with both Ms. Winfrey and late-night rival Jay Leno -- after Mr. Letterman had publicly called into question the NBC host's decision to take his "Tonight Show" back from Conan O'Brien.

Now, as it turns out, "The Late Show" host may be gearing up for a three-peat. In a quick remark last week at a CBS media event about the network's Super Bowl plans, CBS CEO Leslie Moonves strongly hinted that Mr. Letterman may once again serve in a Super Bowl slot. Securing the wee-hours comedian's services, however, has previously proved to be "a pretty tall order," Mr. Moonves said then. "Whether we can pull it off we don't know yet."

That's partly because Mr. Letterman owns production of his show, giving CBS perhaps less influence over its late-night star than, say, NBC has over Mr. Leno. Mr. Letterman is also not exactly obsequious to the networks that have aired him.

But the two sides -- CBS and Mr. Letterman's production company, Worldwide Pants -- are definitely in discussions about what, if anything, to do during the event. CBS considers Mr. Letterman's attention-grabbing appearances to be something people have come to expect, said George Schweitzer, CBS's marketing president, in a brief interview with Ad Age, and "we hope the tradition continues."

A spokesman for Worldwide Pants said executives declined to comment. No surprise there. Mr. Letterman's Super Bowl promos have worked so well partly by sneaking up and catching you off guard. If the network were to announce officially Mr. Letterman's presence in the game -- or hint what he might be up to -- it would be the equivalent of removing a shark's teeth before they chomped into unsuspecting prey. It's actually surprising that Mr. Moonves revealed as much as he already did.

Mr. Letterman's Super Bowl work, both past and potentially present, also spotlights the shifting importance of promos to the big broadcast networks. Essentially in-house ads for each network's TV shows, the venerable ol' promo is starting to benefit from new thinking by TV executives. Why air the usual clips of scenes from the shows when you can make a larger statement to a Super Bowl crowd of 100 million-plus, many of whom haven't tuned in to the program being promoted? CBS, NBC and Fox have all gone the extra mile in recent Super Bowls, running promos that make better use of humor and celebrity and stand much closer to the high-production-value ads from paying sponsors.

One can make the argument that the CBS Letterman promos rank up there with the best of the Super Bowl: Apple's "1984" ad; Google's poetic "Parisian Love" spot; or Chrysler's recent America-in-revival work with Clint Eastwood. These commercials catch people by surprise with a simple but bold idea and beautiful execution.

Part of the appeal of the Letterman work is its easy feel. Each shows Mr. Letterman sitting on a couch watching football. The surprise comes after viewers discover who is sharing the sofa. In a time when so many Super Bowl ads are previewed endlessly in the weeks before the Big Game, the CBS promos have provided nice curve balls, showing feuding personalities putting aside their differences to enjoy the simple pleasures of watching a big sports event.

In that vein, Mr. Letterman's couch -- if it surfaces during the Super Bowl at all -- could this year include anyone from Sarah Palin to Paris Hilton, to name just two people Mr. Letterman has upset with his "Late Show" antics. But the pressure would no doubt be on to top the visit from Mr. Leno and Ms. Winfrey in 2010.

What guest might top that stunt? Viewers will have to makes sure to stay by the screen, because a run to the bathroom, a glance at a mobile tablet or a dash for buffalo wings could mean missing a seminal moment in this year's Super Bowl advertising game.

Tuning In is a series of commentaries by Ad Age TV Editor Brian Steinberg on the TV schedule, the ads it carries and changes within the industry. Follow him on Twitter.