Network hardware giant Cisco is thinking about it. They believe that digital signage will become a $2 billion market by 2010. Keep in mind that "digital billboard" can include signage with smaller dimensions. That means that the same sign you saw on the highway might also be placed in the lobby of your hotel and on your screen when you turn on the TV in your room. Cisco acquired Tivella, Inc., a leading provider of digital signage software and systems, in December of last year.

Google is thinking about it. Last December, the search leviathan (bigger than the merely "giant" Cisco) filed with the US patent office for "allocating advertising space in a network of displays." What they are proposing sounds exactly like a version of Google AdWords for digital billboards.

Given the way that AdWords already works, an advertiser will be able to specify the message and the graphics for a campaign that stretches from the desktop to the highway with just a few clicks.

If you are wondering if these signs are a distraction, they certainly are. However, did I mention that these signs can also show animations or even short film clips? Now, that's distracting.

But don't worry. Advertisers, I mean. Because digital ads shown on flat panel displays mounted on the ceiling of ambulances mean that you will never lose touch with your customer, who will also see the same ads on the TV in his intensive care room, step-down facility, and then in the lobby as he goes home.

There is just one additional technology yet to be employed. In 1963, science fiction writer J.G. Ballard wrote a great short story, The Subliminal Man, that predicted the use of enormous electronic signs to convince you to BUY NOW:

Franklin paused, looking up at the huge metal sign mounted in an enclosure at the edge of the car park. Unlike the other signs and billboards that proliferated everywhere, no attempt had been made to decorate it, or disguise the gaunt bare rectangle of riveted steel mesh. Power lines wound down its sides, and the concrete surface of the car park was crossed by a long scar where a cable had been sunk... A dim but powerful humming emanated from the transformers below the sign, fading as he retraced his steps to the supermarket...
"You've been rambling about these signs for weeks now. Tell me, have you actually seen one signaling?"
Hathaway tore a handful of leaves from the hedge, exasperated by this irrelevancy. "Of course I haven't, that's the whole point, Doctor..."
(Read more about subliminal billboards)

If I sound less than enthused about digital signage, see my earlier article Ad Saturation Approaches 100 Percent.

And yes, there are plenty of other science fictional precursors for out-of-control advertising on every possible surface; don't miss the projection commercials from a 1974 novel by Alfred Bester. You'll also wait breathlessly for airplane window ads from Pohl/Kornbluth's 1952 novel The Space Merchants.

Here are some other articles about the future of advertising: French Billboards Call Your Cellphone, A-170 Video Lightsign Airship Brings Bladerunner Ad Blimp To Sky Near You and Sky Billboards In Fact And Fiction.

I'm not through scaring you yet. What about digital billboards that change just for you? See BluScreen Minority Report Ads, which are actually being tested as we speak.