Agency executives are of two minds when it comes to mobile. On the one hand, they see a potentially huge market. On the other, it's one that's still quite small. Tuesday's Mobile Ad Summit in New York, part of Advertising Week, attracted several top agency chiefs, including IPG CEO Michael Roth. And the message was that mobile is a big part of the marketing world's future, even if it isn't a big part of the bottom line today.

Reuben Hendell, CEO of IPG digital shop MRM, said his agency is formalizing its mobile practice as the MRM Mobile Net. All together, mobile is now 10 percent of MRM's revenue, he said. Matt Seiler, global CEO of fellow IPG agency Universal McCann, said mobile hasn't become as big of a business for UM, estimating that it accounts for less than 5 percent of revenue. "It's a great platform that we look forward to doing something with, but we're not there yet," Seiler said. Roth is bullish on mobile for the same reason Willie Sutton was fond of banks: It's where the money is—or will be. He cited a study that found one-third of people would prefer to give up sex for a year than their mobile phone. "That will give you an idea of how important [mobile] is," Roth said.

On the creative side, apps and location-based services provide yet another way to reach customers with marketing that is less interruptive and more of a service. Scott Prindle, executive creative technology director at Crispin Porter + Bogusky, said he believes mobile has the potential to act as "the glue" for campaigns, connecting the digital and physical worlds. In some cases, advertising ideas begin in mobile. CP+B's recent work for the carrot industry includes a mobile application that's a video-game controlled by the crunching sound of biting baby carrots. (See the trailer below.) The game ended up influencing a TV spot.